The primary role of the Canadian Geodetic Survey (CGS) is to define, maintain, continuously improve and facilitate efficient access to the Canadian Spatial Reference System (CSRS) through various geodetic tools and products data. Accurate positioning (latitude, longitude and heights), once possible only through traditional survey methods and tied to geodetic points in the ground, has now been made possible thanks to space-based technologies. As a result of this, good spatial referencing will help ensure that mapping, navigation delimitation of property and other georeferencing needs all relate to a common spatial reference system.
Canadian Spatial Reference System (CSRS)
The Canadian Spatial Reference System (CSRS) is a three-dimensional grid on which positions (latitude, longitude and height) of any object or feature can be precisely pinpointed. The infrastructure underlying a grid consists of a network of points whose coordinates are determined with the highest precision. Grids are fundamental for mapping, marine charting, navigation, boundary demarcation, crustal deformation study and other georeferencing applications.
At the heart of the CSRS is the Canadian Active Control System (CACS); a network of continuously operating GNSS receivers. CACS data support positioning accuracy at the decimeter-level (e.g. for imagery geocoding and realtime applications), the centimeter-level (e.g. for legal surveys) and the millimeter-level (e.g. for measuring crustal motion).
Geodetic networks traditionally consisted of control monuments distributed across our landmass for surveyors to occupy and access the geodetic grid as well as control their surveys. In the early 1980’s, with the advent of Global Navigation Satellite System (GNSS), geodetic control became accessible from space with great accuracy. CGS established the Canadian Active Control System (CACS) to continuously track GNSS satellites and compute their precise orbits. Several networks of Active Control Points have been added across Canada for various monitoring purposes. For example, the CSRS is tied to the world gravity network with an Active Control Point (ACP) at the Canadian Absolute Gravity Station (CAGS).
The extension of the geodetic reference frame into space also brought along the requirement for monitoring the Earth’s orientation in space which is provided by observations of quasars with Very Long Baseline Interferometry (VLBI). These activities marked CGS’s entry into the space-age of precise geodetic positioning.
In the mid-1990’s a program to install and observe stable monuments, known as the Canadian Base Network (CBN), was initiated. The CBN was deployed at 200 km spacing in southern Canada and 500 km up north, for multi-epoch high-accuracy GNSS positioning. Initial observations of the CBN revealed the presence of distortions of up to 2 meters in the original realization of our national grid, the North American Datum of 1983 (NAD83). Over the past decade, re-observation of the CBN network also confirmed that crustal motion of the order of a few millimeters per year horizontally and up to a couple of centimeters vertically is occurring Canada-wide. The CBN network also provides anchor points for the integration of denser provincial high-precision networks.
Today, CGS supports users of space-based technologies with a stable, although sparse ground infrastructure, precise orbit products, gravimetric geoid models and the tools to facilitate access to the Canadian Spatial Reference System (CSRS).
North American Datum of 1983 (NAD83)
The current reference system adopted as a national georeferencing standard by most federal and provincial agencies in Canada and endorsed by the Canadian Council on Geomatics [CCOG 2006] is the North American Datum of 1983 (NAD83).
NAD83(CSRS) represents the 3D realization of the original NAD83, constituted by traditional horizontal network hierarchy. This new reference frame hierarchy is divided into active and passive components as illustrated in Figure 1. The active component consists of networks of continuously operating GNSS receivers and products derived from them, such as precise orbits and broadcast corrections. The passive component is comprised of more traditional monumented control points that users can occupy with their own equipment.
Very Long Baseline Interferometry (VLBI)
What is Very Long Baseline Interferometry?
VLBI, or Very Long Baseline Interferometry is a technique used to determine very precise distances between radio telescopes in order to study the Earth, its place in the universe and to monitor the changes in both. VLBI uses radio signals from deep space such as quasars to measure how the continents are moving, how the spin rate of the Earth is changing, the motion of the pole and even how the Earth's axis wobbles in space.
How does Very Long Baseline Interferometry work?
VLBI uses two or more radio telescopes to observe and record the signals received from the same quasar at exactly the same time. The time difference between the arrival of the signal at each radio telescopes can then be used to calculate a very precise distance between the telescopes. VLBI can determine distance between radio telescopes to within a millimeter across an entire continent.
VLBI observes radio energy emitted by quasars. A quasar is an enormously bright object at the edge of our universe. The word quasar, short for "quasi-stellar radio source" was named in the 1960's when quasars were first detected. A quasar viewed with an optical telescope appears point-like and similar to a star, but is actually quite large and gives off energy a trillion times brighter than the Sun. Quasars are so far away, that even as bright as they are, they cannot be seen without a very powerful telescope. Indeed, quasars are the most distant objects yet detected in the universe.
Throughout history, mankind has used the stars for navigation and to study the Earth. Unfortunately, using nearby stars in our galaxy has limitations. First of all, the Earth's atmosphere refracts light in unpredictable ways. This limits our ability to precisely position any star. Secondly, all the stars visible to us are not fixed in space. They move in different directions and, worse than that, at different speeds. Trying to accurately position a point on Earth using moving stars can be compared as attempting to position yourself on a boat by using all the other moving boats out on the water. What we need instead of other boats, is a fixed point on the shore to use as a beacon. Quasars are therefore the "beacons" that we can use to position ourselves in the universe.
How does a quasar act as a beacon?
First of all, because the radio waves from quasars are virtually unaffected by atmospheric refraction, we can determine their position very precisely. Secondly, quasars are the farthest known cosmic objects. Energy from a quasar can take 10 to 15 billion years to reach the Earth's atmosphere. Quasars are so far away that they never appear to move. This allows us to determine their position very well. Indeed, quasars have proved themselves to be very reliable "beacons in the sky". Not only that, but because quasars are scattered in all directions and since radio signals are unaffected by our atmosphere, quasars are a very convenient way to position ourselves in the universe, day or night, rain or shine and can be observed from all around the world.
How do we benefit from VLBI?
Much of what we know about the interior of the Earth has been gained through indirect observations. The size of the Earth, its shape, the changing orientation of its polar axis and varying spin rate have all been determined by observing the stars and have played a major role in our present understanding of the Earth's structure.
VLBI produces very precise distance measurements on the Earth's surface and assists us in our understanding of its interior, atmosphere and oceans. For example, it was found that the 1997 El Niño weather system lengthened our day by 0.6 milliseconds. Indeed, major weather systems like El Niño can literally cause the Earth to speed up or slow down. What about continental drift? North America and Europe are drifting apart at a rate of about 1 cm per year.
Another benefit of VLBI is its capability to accurately determine our position in the universe. Just as the Earth rotates around the Sun, the Sun rotates around the Milky Way Galaxy. The 100 billion stars including the Sun making up the Milky Way Galaxy all move around the Galaxy at different speeds. Despite all this motion, VLBI observations make it possible to fix our position in the universe.
Why Geodesy by VLBI?
Modern Geodesy relies on astronomical and space-based techniques, namely VLBI, GNSS, SLR (Satellite Laser Ranging) and LLR (Lunar Laser Ranging).
IAG commission VIII (International Coordination of Space Techniques for Geodesy and Geodynamics (CSTG)) discusses the different space techniques in its status report for 1996, and argues that "Only VLBI is capable of defining the Celestial Reference Frame, which serves as the inertial system in satellite geodesy. Long-term stability of UT1-UTC and of precession/nutation can only be guaranteed by this technique." Their summary states: "It is clear from the scientific point of view that VLBI, Laser Ranging (SLR and LLR), and satellite microwave techniques are indispensable as contributors to space geodesy."
The precise measurements made possible by VLBI observations make it possible to detect even the slightest changes in the orientation of the Earth in space.
- Planetary gravitational attraction
- Ocean currents
- Atmospheric pressure
- Melting of ice
- Ocean loading
- Continental water
- Post-glacial rebound
- Lunar/solar gravitational tides
- Plate motion
- Mantle convection
- Core topography
- Electromagnetic forces
- Viscous torques
Canadian Active Control System (CACS)
Active Control Points (ACPs)
Active Control Points (ACPs) are similar in concept to the most accurate points of the older ground-based control networks. The positions and elevations of the ACPs have been very precisely and accurately determined. Each of these unattended stations use a very high quality GNSS receiver to constantly track all the GNSS satellites it can see or "view" and record data sent by radio signal from each satellite 24 hours a day, 7 days a week, all year long.
You can view and download Active Control Point station descriptions from the CACS interface.
Definition of the CACS
The Canadian Active Control System (CACS) is a national network of continuously operating geodetic quality GNSS receivers. It comprises of a Master Active Control Station (MACS) and a network of continuously operating GNSS data acquisition stations, called Active Control Points (ACPs), which are distributed across the Canadian landmass. Each GNSS tracking station provides dual-frequency pseudorange and carrier phase measurements to a central computer where GNSS corrections are generated.
What does it provide?
The CACS was created to improve the precision and efficiency of GNSS positioning in Canada and to provide easy access to the Canadian Spatial Reference System (CSRS), the national reference system used throughout Canada. This is accomplished by monitoring GNSS integrity and performance from the analysis of data acquired through continuous tracking; by computing and making available precise satellite ephemerides (GNSS orbits) and precise satellite clock corrections; by supporting Wide Area Differential GNSS (WADGNSS) development and other applications (geodynamics, precise time transfer, etc)
If a user is positioning with GNSS, the use of the Canadian Active Control System (CACS) provides a tie between modern positioning systems (such as GNSS) and a common reference system such as the Canadian Spatial Reference System.
How does it work?
The system consists of unattended GNSS tracking stations, the Active Control Points (ACPs), which continuously record carrier phase and dual-frequency pseudorange measurements in real-time at 1 Hz for all satellites of the Global Navigation Satellite System (GNSS) within station view. Each ACP is equipped with a high precision dual-frequency GNSS receiver and an atomic frequency standard. Temperature, pressure and humidity data are also collected at selected ACP sites. The data collected at each ACP is retrieved on a daily basis by a central processing facility in Ottawa, where wide area GNSS corrections are generated.
Functionality of the CACS
Active Control Points (ACPs)
The availability of precise ephemerides, precise satellite clock corrections and observational data from the ACPs offers significant benefits for Canadian users carrying out GNSS surveys. These Active Network make it possible to position any point in Canada with a precision ranging from a centimetre to a few metres in relation to the national spatial reference frame without actually occupying an existing control monument or base station.
Positioning at the metre level from pseudorange (code) observations without the use of a base station is made possible by using CACS precise satellite clock corrections. They can be applied anywhere in the world to correct the users observed ranges and, when used along with CACS precise ephemerides, provide positioning accuracies in the 1 metre range depending on the user's receiver measurement noise and multipath. The advantage of using satellite clock corrections is that some of the station specific errors can be accounted for directly, as opposed to assuming commonality of errors between base station and remote site as with local GNSS. Since the satellite corrections are based on a network of accurately known reference points, some of the uncertainty associated with using control data from a single base station is effectively removed. The distributed stations of the CACS and the International GNSS Service (IGS) networks also ensures common satellite visibility for any user at any time of day, eliminating the problem of 'matching' observations between remote and reference sites sometimes associated with Local Area Differential Global Navigation Satellite System (LDGNSS) operations.
For survey projects using GNSS phase measurements and requiring the highest precision, introducing CACS precise ephemerides in the data processing will reduce all orbit related errors in GNSS baseline determinations to less than 0.1 ppm. These errors can reach 3 parts per million or more when ephemerides broadcast by the satellites are used. Furthermore, by including observational data from the ACPs in the data processing, a direct tie to the national spatial reference frame is established. Scale and orientation are provided by the precise ephemerides without occupation of any control points thereby increasing the efficiency of field operations and data processing. Depending on the GNSS software, further advantages may be realized by using precise ephemerides, such as improved cycle slip detection and fixing capability, enhanced carrier phase ambiguity resolution, better and more consistent aposteriori error estimates, etc. Recent tests, combining CACS data and precise ephemerides, has shown static positioning precision at the centimetre level in each of the three-dimensional components for distances up to 600 km when appropriate software and adequate procedures are applied.
International contribution and participation
The Canadian Geodetic Survey also contributes CACS data to the International GNSS Service (IGS) and participates as an analysis centre, therefore having access to data from globally distributed fiducial sites for use in the computation of precise satellite ephemerides. Through the IGS, CACS data and related products are made available to international organizations such as the International Earth Rotation Service (IERS), the NASA Crustal Dynamics Data Information System (CDDIS), the US National Geodetic Survey (USNGS), the US Naval Observatory (USNO) and other organizations interested in Earth dynamics. The precise observations of the satellites made from the fiducial stations are used to establish the Earth Orientation Parameters (EOP) and derive inter-station baseline lengths and orientation for regional monitoring stations. Changes in baseline components over time provide quantitative data for studies of geodynamics, natural hazards and global change.
Passive control networks
Modern passive control networks
Canadian Base Network (CBN) Go to interface
Initiated in 1994 in order to complement the CACS, the Canadian Base Network (CBN) provides the link between the existing land-based control networks and the CACS. It has been designed to serve as the new ground-based portion of monumented survey control in the Canadian Spatial Reference System (CSRS).
The CBN is a network consisting of pillar monuments with forced-centering plates, positioned 3-dimensionnally with GNSS to centimetre-level accuracy respective to the CACS. The network consists of an array of pillars at an average spacing of 200 km in the built-up areas of southern Canada, 500 km in the middle regions of Canada, and 1,000 km in the northern areas.
As well as being a GNSS control network, the CBN can serve as a monitoring network for deformation studies of the Canadian landmass.
High Precision 3D Geodetic Networks (HPNs) Go to interface
This extended network consists in approximately 3,000 (non-CBN) stations that were observed with high-accuracy GNSS and tied to NAD83(CSRS) at epochs 1997 or 2002.
You will find 3-D NAD83(CSRS) coordinates for:
- Vertical benchmarks on which GNSS was performed to verify the accuracy of geoid models and help produce the HTv2 height transformation.
- Various older monuments originally tied using traditional survey methods and re-observed by GNSS.
Conventional passive control networks
Horizontal and vertical control networks were developed independently from one another. That is, a survey monument, which provided accurate horizontal coordinates, was not normally connected to the vertical network, nor were vertical points related to the horizontal reference network.
Also, vertical control points were established by using differential or "spirit" levelling, whether stations making up the national horizontal control network were originally established using classical surveying techniques, such as triangulation.
Vertical Control Network (CGVD28 and CGVD2013 ) Go to interface
Over the past one hundred years, a national network of vertical control points or "benchmarks" has been created by levelling along major roads and railways in Canada and by linking more than 30 tide gauges operated by the Fisheries and Oceans Canada.
The Canadian Geodetic Vertical Datum of 1928 (CGVD28) was realized by doing a national scale adjustment of all levelling at the time. Since then, new levelling was added and adjusted to the old in a piece-meal approach that inherently left distortions throughout the network. NRCan no longer expand nor maintain the CGVD28 network of benchmarks.In November of 2013, CGVD28 was replaced by CGVD2013, which is a new geoid based datum for Canada. You can find more information on CGVD2013 here.
Horizontal Control Network (NAD83) Go to interface
A horizontal framework of interconnected control surveys, with monuments (physical markers in the ground) spaced from 20 km up to a maximum of 100 km apart, extends over the entire Canadian landmass. These control stations, despite having accurate positions, were often inconvenient to use due to restricted or limited access or inter-station visibility. With the development and increased use of commercial GNSS receivers and software, it is possible and financially more beneficial to produce much better data than was previously obtainable from these earlier classical methods.
Northern Horizontal 2D Network
Part of the Canadian spatial reference system is an horizontal 2D network of geodetic monuments spaced 20 to a 100 km apart in the Yukon, Northwest Territories and Nunavut. Station coordinates were determined using a variety of survey methods (non-GNSS) and accuracy varies. Most have horizontal coordinates labelled simply "NAD83" signifying they are NAD83 (Original) and compatible with NAD83(CSRS) but at the meter level or worse.
The Northern Horizontal 2D Network is included in the interface of the Horizontal Control Network (NAD83).
Canadian Gravity Standardization Network (CGSN)
Some 1,400 primary and secondary control stations, systematically distributed throughout Canada, represent the realization of the current Canadian Gravity Standardization Network (CGSN). The adjustment of the CGSN is based on the International Gravity Standardization Network of 1971 (IGSN71). Therefore, the datum definition is considered accurate to several tens of microGals (1 microGal = 10-8 m s-2).
Since the late 1990’s, the Canadian Geodetic Survey (CGS) reduced significantly the maintenance of the CGSN in favor of absolute gravimetry. Today, CGS regularly maintains 68 absolute gravity stations distributed across Canada. These stations are augmented by another 77 absolute gravity stations that CGS or Geological Survey of Canada (GSC) observed for specific scientific projects (some of these stations are still active today). The accuracy of the absolute gravity stations is a few microGals. CGS participates regularly in international and continental comparisons of absolute gravimeters for calibration purposes (to validate measurement standards).
Gravimetry and Geodesy
The three "pillars" of geodesy include:
- The shape of the Earth (Geokinematics)
- The Earth’s rotation
- The Earth’s gravity field
Gravity measurements provide knowledge about the Canadian landmass. Gravity helps geodesists improve national surveying standards (e.g., levelling) and enables the computation of the geoid with respect to the ellipsoid, allowing 1) the depiction of the actual shape of the Earth and 2) the relation between the GNSS ellipsoidal heights and orthometric heights (often referred as heights above mean sea level). It can also help identify potential oil and mineral-bearing geological formations, understand Earth physical processes, and define Canada's continental margins in the sovereignty issue over the North Pole.
- Otto J. Klotz was the first to use the original Mendenhall pendulum gravimeter in Canada in 1902. He acquired his first readings in Ottawa and later Montreal and Toronto to demonstrate the utility of gravimetry to the Dominion Observatory.
- The first regular gravity survey was conducted in 1914-15 and consisted of 18 points. As Canadian activities in gravimetry flourished, a National Gravity Program was created to map the gravity field over all of Canada's lands and offshore.
- Following the Mendenhall pendulum came the torsion balance, the relative gravimeter and finally the Absolute Gravimeter (AG) that directly determines acceleration due to gravity by precisely measuring the time and distance travelled by a free-falling optical mass in a vacuum chamber.
- NRCan moved gravity activities from the Earth Physics Branch to CGS in 1995.
CGSN Modernization and the Canadian Absolute Gravity Array
In support to the vertical component of the CSRS, CGS established an array of Absolute Gravity (AG) observation sites co-located with GNSS reference sites across Canada (68 stations). These sites allows CGS and NRCan’s partners to conduct scientific studies that include tectonic plate and earthquake deformation, sea-level rise, hydrological mass monitoring (glaciers and water) and glacial isostatic adjustment. The new Canadian Absolute Gravity Array is replacing the primary control points of CGSN.
A future project is the readjustment of CGSN with constraints to the absolute gravity stations.
Study of the Earth's Gravity Field
The tools and methods used to study the Earth's gravity field range from land-based instruments to space-based systems, capable of measuring unbelievably small changes in the gravity field.
Measuring the Earth's Gravity Field from Land
On land, CGS makes use of three types of instruments to measure gravity, gravity difference and gravity variation: absolute gravimeter, relative gravimeter and superconducting gravimeter. Using measurements from these instruments, CGS can map the gravity field and monitor its geographical and temporal variation across the Canadian territory.
Measuring the Earth's Gravity Field from Space
Four revolutionary satellite gravity missions are further advancing the study of the Earth’s gravity field from space:
- CHAMP (CHAllenging Minisatellite Payload) 2000-2010
- GRACE (Gravity Recovery And Climate Experiment) 2002-2017
- GOCE (Gravity field and steady-state Ocean Circulation Explorer) 2009-2013
- GRACE-FO (Gravity Recovery And Climate Experiment Follow-On) 2018-
The satellite gravity missions contribute in improving the long and mid wavelength components (down to 200 km) of the Canadian geoid model, which is now the basis for Canada's new height reference system (CGVD2013). Furthermore, satellite gravimetry can measure the temporal variation of the gravity field (300-km wavelength) by monitoring mass redistribution coming from, for example, glacial isotactic adjustment, tectonic plate motions, large earthquakes, glacier melting and hydrological changes (e.g., groundwater variation).
Why use gravity?
On Earth, accurate charting of variations in gravity have been used for:
- Mining exploration - Prospecting for minerals, oil and gas reserves
- Volcanology - Predicting volcanic activity
- Oceanography - studying and monitoring the state of average mean sea level is used to show changes in ocean currents and even provide us with advance flood warnings
- Navigation - Inertial navigation systems of planes, ships, and missiles
- Geophysics - Studying the Earth's interior – gravity provides key information on core and mantle dynamics, which can be used to explain the Earth's geology
- Glaciology - Precise estimates of the thickness of polar ice sheets
- Satellite orbit prediction - accounting for the impact of gravity on the orbit of a satellite means better performance from the many satellites orbiting the Earth and their associated technologies, such as, GNS positioning
- Surveying and mapping - accurate gravity observations are used to make a model of mean sea level (geoid)
- Manufacturing and medicine - shuttle missions and planned space station research will study cell design and manufacturing processes in a zero gravity environment in an effort to discover a cure for diseases, such as cancer, or to build better machines, even a new mousetrap
- Curiously, GNSS is also an important tool in studying and advancing our knowledge of the Earth's gravity field
Factors Affecting the Gravity Field
Gravity is acceleration caused by two forces: gravitational (mass) and centrifugal (rotation). The latter represents only a maximum of about 0.3% of the gravity at the equator where Earth’s rotation is the fastest. For most people, gravity appears static, and is often approximated to 9.8 m/s2. In fact, it is changing because Earth is not a homogeneous sphere, masses are being redistributed continuously and the rotation is slowing down ever so slightly. Naturally, humans do not feel changes in gravity, but modern instrumentation can measure minute changes in the gravity down to the nanoGal (10-11 m/s2). By measuring these small changes in gravity, it allows scientists to have a better understating of the Earth’s physical processes and hydrological cycle.
At any given moment in time (or when averaging over a specific period) the gravity field differs with location and depends on several factors:
- The most rapid changes in gravity is with altitude (H). Gravity decreases with elevation at a vertical rate of 0.3 milliGals/m (1 milliGal = 10-5 m/s2).
- Gravity changes with the latitude (Φ) because Earth is an oblate spheroid (a squished sphere). As the poles are closer to the Earth mass center than the equator, gravity increases from the equator to the poles by approximately 5000 milliGals (0.05 m/s2). This represent a horizontal rate (north-south direction) of 0.5 microGals/m (1 microGal = 10-8 m/s2).
- Large scale gravity variations are related to Earth’s topography (mountains, ocean trenches, etc.). This can represent changes on the order of 100 milliGals (0.001 m/s2).
- Variation in gravity is associated with density (g/cm3) difference in the Earth’s crust and mantle. The magnitude can be tens of millGals (10 milliGals = 0.0001 m/s2).
As the Earth is a dynamic planet, gravity has a temporal variation. It can be organized into two groups: variations created by slow processes that operate over geological time scales (hundreds to billions of years), and variations created by processes that operate on shorter timescales as large as decades and as small as hours or days. The former creates what we consider to be the static gravity field – it changes too slowly to be observed. The latter creates what we consider to be the transient gravity field – changes are observed and provide important information on modern day climate and environmental changes.
Factors that result in observed changes in gravity over time scales of hundreds of years or less:
- Changes in surface, near-surface, or ground water – even small changes in water over large areas have a measurable effect on the gravity field – annual and long term trends are detected and monitored
- Melting of glaciers – many glaciers are melting rapidly and this change in mass distribution has a significant impact on Earth’s gravity
- The moon – the gravitational force associated with the moon orbiting the Earth results in ocean tides as well as solid Earth tides (the solid Earth deforms as well!) – both of these transient mass redistributions affect gravity
- Earthquakes – earthquakes redistribute mass locally and change the local gravity field
- Extraction or injection of fluids – extraction of petroleum products or injection of fluids result in local changes in gravity
- Glacial rebound – regions that were once forced to sink due to the weight of massive overlying glaciers are now, with the glaciers no longer there, slowly returning to their pre-glacial equilibrium – this results in a sizeable vertical uplift across Hudson Bay and Greenland and affects the large majority of Eastern and Northern Canada.
- Sea-level change – sea-level rise adds mass to the oceans and removes it from land – this redistribution of mass affects the gravity field. In fact, sea-level change does not occur to the same extent everywhere. This is because the surface height of the ocean is determined by local gravity. Significant melting of a glacier will add water to the oceans – globally increasing sea-level on average. But near the glacier, the loss of mass from the glacier will reduce the local gravity field enough that sea-level actually falls. As such, sea-level change and gravity are closely coupled.
The list of factors affecting the gravity field can be infinite as any mass redistributions change gravity, though many of them are often too small to be measured with current instrumentation.
You can access the CGSN and absolute gravity stations via the Gravity Network (CGSN) interface.
Geodetic Reference Systems
International Terrestrial Reference Frame
Since 1990, the most accurate and stable reference frames available are the successive versions of the International Terrestrial Reference Frame (ITRF) produced by the International Earth Rotation and Reference Systems Service (IERS).
The ITRF reference system is global and as such is not fixed to any specific tectonic plate. The specific ITRF frame used by PPP is the one realised by IGS at the epoch for which the precise GNSS orbits were computed.
A key difference with previous reference systems is the dynamic nature of the reference frame. Coordinates for stations are valid for a specific date (epoch) and are accompanied by velocity estimates for propagating coordinates to other epochs.
- Individual realizations are denoted by ITRFxx, where xx represents the last year for which data was included in a particular solution.
- ITRF coordinates of points are constantly changing due to the motions of the individual tectonic plates. It is therefore necessary to specify which epoch ITRF coordinates refer to and to account for tectonic motion when propagating coordinates to other epochs.
North American Datum of 1983
The North American Datum of 1983 (NAD83) is the national spatial reference system used for georeferencing by most federal and provincial agencies in Canada. The NAD83 system is based on a global reference system known as the BIH Terrestrial System 1984 (BTS84) together with the reference ellipsoid of the Geodetic Reference System 1980 (GRS80).
The physical realization of this system has undergone several updates since it was first introduced in 1986. It has evolved from a traditional, ground-based horizontal control network to a space-based 3D realization fully supporting more modern positioning techniques and the integration of both horizontal and vertical reference systems.
Unification of NAD83 in Canada and the U.S. via the ITRF
In order to provide a more precise realization of a 3D NAD83 common to both Canada and the United States, and to comply with IAG resolutions recommending that reference systems be tied to the ITRS, a new common NAD83 transformation was derived with respect to ITRF96, the most recent at the time. A conformal 3D seven-parameter similarity (Helmert) transformation used the NAD83(Original) and ITRF96 coordinates at epoch 1997.0 at 12 VLBI stations in both countries.
The scale of ITRF96 was adopted for this realization of NAD83 by setting the scale parameter to zero after estimation. This ensures the scale of NAD83 will be compatible with the more accurate scale defined by ITRF96 and used by other systems such as WGS84.
Tectonic Plate Motion
Continuous observations at GNSS reference stations worldwide permits the monitoring of tectonic plate motion. The North American Tectonic plate rotates in a counter-clockwise direction at a rate of approximately 2 cm per year. Two regions of Canada experience major regional seismic and tectonic activity: the West Coast and the Lower St-Laurence valley.
In order to correctly account for the tectonic motion of the North American tectonic plate when transforming from/to ITRF96 positions at any arbitrary epoch, the NNR-NUVEL-1A plate motion model was adopted as recommended by the IERS.
North American Datum of 1983 (CSRS)
The North American Datum of 1983 (Canadian Spatial Reference System) is the improved realization of NAD83 for Canada.
It represents the 3D realization of the original NAD83, constituted by traditional horizontal network hierarchy. This new reference frame hierarchy is divided into active and passive components as illustrated in Figure 1. The active component consists of networks of continuously operating GNSS receivers and products derived from them, such as precise orbits and broadcast corrections. The passive component is comprised of more traditional monumented control points that users can occupy with their own equipment.
Its main advantage is that it provides almost direct access to the highest level of the NAD83 reference frame through ties to the CACS and collocated VLBI stations that form part of the ITRF network. These stations effectively act as both ITRF and NAD83 datum points for geospatial positioning, thereby enabling more accurate, convenient, and direct integration of user data with practically no accumulation of error typically found in classical horizontal control networks. Moreover, GNSS orbits can be transformed to NAD83 allowing users to position themselves directly in NAD83 through applications such as Precise Point Positioning (PPP).
World Geodetic System 1984
The World Geodetic System 1984 (WGS84) is a global reference frame originally developed by the US Defense Mapping Agency (subsequently renamed the National Imagery and Mapping Agency (NIMA) and now called the National Geospatial-Intelligence Agency (NGA)).
The Geoid model contributes to the vertical component of the reference system so that ellipsoidal GNSS heights can be converted to orthometric elevations for practical uses.
The real challenge lies in knowing the relationship between the ellipsoid and the geoid. Once we determine the difference between these two surfaces, called the "geoid-ellipsoid separation" or "geoidal height", at a given point, we can then apply the geoidal height to our GNSS height measurement to get the mean sea level elevation.
There are three key elements to consider when examining the geoid:
- Topography: The surface of the Earth
- Ellipsoid: GNSS heights are referenced to this mathematical surface
- Geoid: The natural surface extension of mean sea level
How accurate is the current geoid model?
It varies according to the region. In Canada's Rocky Mountains, there are greater variations in the Earth's gravity field and fewer gravity measurements available. The geoid is not as accurate there as in the Prairies or Eastern Canada where the Earth's gravity field is more consistent and better defined. In general, though, the accuracy of our current geoid with respect to the center of the Earth is +/- 5 cm. It differs from coast to coast by about 40 cm. The more accurately we can determine the geoid, the more accurate heights we can get using GNSS resulting in easier, faster and cheaper construction of highways, sewers and other projects.