Planimetry consists of the identification and geolocation of basic land cover (e.g. forest, marsh), drainage, and anthropogenic features (e.g. urban infrastructure, transportation networks) in the x, y plane. Planimetric information is generally required for large-scale applications - urban mapping, facilities management, military reconnaissance, and general landscape information.
Why remote sensing?
Land surveying techniques accompanied by the use of a GPS can be used to meet high accuracy requirements, but limitations include cost effectiveness, and difficulties in attempting to map large, or remote areas. Remote sensing provides a means of identifying and presenting planimetric data in convenient media and efficient manner. Imagery is available in varying scales to meet the requirements of many different users. Defence applications typify the scope of planimetry applications - extracting transportation route information, building and facilities locations, urban infrastructure, and general land cover.
Very high resolution is usually a requirement for accurate planimetric mapping. Concerns of the mapping community with regard to use of satellite data are spatial accuracy and the level of detail of extractable information content. The concern for information content focusses not only on interpretability of features, but on the ability to determine the correct spatial location of a feature. An example of the latter would be the difficulty associated with defining the centre of a river or precise location of a powerline or pipeline right-of-way in vector format, when interpreting from a relatively coarse raster base. Spatial resolution is a critical element in this case.
The turnaround time of one or two weeks will generally meet the requirements for this type of mapping, although defence requirements may be more stringent.
Canada vs. International
For general Canadian applications, the ability to provide planimetric information is best addressed by current VIR sensors, and for large scale mapping- aerial photography. The importance of adequate resolution and information content outweigh the need for near real time products. Presently, TM and SPOT data provide optimal information for extracting planimetric information for regional applications. Air photos, and particularly orthophotos when available, are preferred for smaller, well defined areas.
For cloud covered areas, radar is the obvious choice for providing planimetric data. The detectability of linear features improves when they are oriented perpendicular to the radar look direction. This can be controlled with airborne sensors, by planning the flightlines appropriately. Another issue is that a balance between resolution and speckle has to be reached. Although single look data provides the finest resolution, speckle can be a hindrance to interpretation, and invites multilook processing.