A complete orthorectified Landsat 7 mosaic of the Canadian arctic archipelago

Jack Gibson, Stefan Nedelcu, Goran Pavlic and Paul Budkewitsch, Canada Centre for Remote Sensing, Earth Science Sector, Natural Resources Canada

Figure 1. Landsat-7 Mosaic of the Canadian Arctic Archipelago. Map projection parameters: Lambert Conformal Conic (LCC); Central meridian: 95°W; Projection Origin: 0°N; First Standard Parallel: 49°N; Second Standard Parallel: 77°N; Ellipsoid Model: WGS84

Figure 1. Landsat-7 Mosaic of the Canadian Arctic Archipelago. Map projection parameters:
Lambert Conformal Conic (LCC)
Central meridian: 95°W
Projection Origin: 0°N
First Standard Parallel: 49°N
Second Standard Parallel: 77°N
Ellipsoid Model: WGS84

Larger image

 

This new mosaic is comprised of the best data available for the Canadian Arctic acquired under snow-free conditions between 1999 and 2002 and is now available for free download through Open Government of Canada search tool. Using over 9,000 ground control points and 1,500 conjugate tie points, the equivalent of about 280 images were assembled to create the mosaic using methodology developed at CCRS [1].  All of the multispectral bands are registered to the panchromatic band and the positional accuracy of each pixel is approximately 7.5 m RMS relative to the control points used.  This represents a maximum position error of about 20 m (at 90% confidence level) anywhere within the mosaic.

The Arctic Islands Landsat mosaic is a seamless geographic image data product accurately registered to better than 1:50,000 scale for the north. The processed data includes all of the multispectral bands at 30 m resolution.  This regional compilation contributes supporting thematic terrain information to serve geographic baseline needs, resource mapping efforts, and environmental applications.

Data Selection

The best cloud free data for creating the mosaic were selected from acquisitions during the snow free periods in the summers of 1999-2002. In contrast to the traditional “square scene” data delivery of 185x185 km Landsat frames, the photogrammetric adjustment process employed long strips of data along an orbital track. A total of 115 tracks were used, each one with the equivalent length of one to seven (typically three) standard scenes.

Figure 2. : In the pair of images above, on the left side is a Level 1G product exhibiting nonconformity of the image with the vector outline of a known lake location shown in red. Image on the right side is the result following the ortho-rectification procedure. The image location of the lake matches well with the red vector outline in this product.

Figure 2. : In the pair of images above, on the left side is a Level 1G product exhibiting nonconformity of the image with the vector outline of a known lake location shown in red. Image on the right side is the result following the ortho-rectification procedure. The image location of the lake matches well with the red vector outline in this product.

 
Figure 3.: Full 30 m resolution view of 500 by 500 Landsat pixels from Kanguk Peninsula, Axel Heiberg Island, Nunavut.

Figure 3.: Full 30 m resolution view of 500 by 500 Landsat pixels from Kanguk Peninsula, Axel Heiberg Island, Nunavut.

 

Data Processing Methodology

The uniform error distribution throughout the mosaic was achieved through the application of a simultaneous block adjustment of multiple acquisition paths each containing multiple scenes.  In contrast to these results, the traditional approach of adjusting each scene individually has been found to produce variable position accuracy for each scene as a function of the control points used [1].  As a result of adjusting the satellite ephemeris and attitude data before resampling the data, the final result also succeeds in preserving the radiometric integrity of the data.

The new procedure minimized the accumulation of planimetric errors that accompanies traditional resampling, orthorectification, and geographic registration steps. This process preserves the radiometric integrity of the spectral data. The technique employed produces products that are based on a single resampling step: values in the original data array → final pixel location in geographic coordinates.

Procedure

A correction for the satellite ephemeris and attitude parameters (e.g. roll, pitch, yaw, position and velocity) is calculated to minimize residual errors for all the Ground Control Points (GCP), and Conjugate Points (CNJ). The method of solving for the correction of the ephemeris and attitude values is to use the co-linearity condition for the GCP measurements and the co-planarity condition for the CNJ measurements to derive a least-squares equation. The results of the solution of the least-squares equation are used to update the ephemeris and attitude values of each orbit path.

The GCP and CNJ measurements were determined using the “Phase Correlation” technique that employs two-dimensional Fast Fourier Transform (FFT2D) routines to determine the spatial phase difference between the images being compared. The comparison of a GCP image and the satellite image begins with image patch sized of 512 lines by 512 pixels; each patch dimension is further reduced in size by a factor of two in each step. The process is completed when the patch sizes reach 16 lines by 16 pixels. At this level, relative geographic positions are computed for further evaluation. The residual errors for the Arctic mosaic are given in the table below.

Measurement Type

Number of Measurements

X Errors RMS (m)

Y Error RMS (m)

Intersection Uncertainty RMS (m)

Ground Control Points

9,100

7.5

7.6

 

Conjugate Points

1,500

 

 

3.8

Summary

A new orthorectification procedure was successfully applied to Landsat 7 data for creating a large area mosaic of the Arctic Islands. Results indicate that RMS planimetric positional errors are on the order of 7-8 m for GCPs and 3-4 m for CNJs. With available GCPs limited to about 5m known accuracy, these seamless map products can be considered accurate to about 20 m, or less than one image pixel. An early version of this mosaic was described by [2] and all digital data of this final version, including some three band composite images, are now available for free download through Open Government of Canada search tool. This is currently the best spatially and radiometrically accurate Landsat product for the Canadian Arctic.

Acknowledgements:

This project was completed for the Hydrocarbon Potential of the Arctic Islands Project under NRCan's Secure Canadian Energy Supply Program with additional support from the Government Related Initiatives Program of the Canadian Space Agency.

References:

[1] Gibson, J.R. and S. Nedelcu. 2008.  An improved approach for the production of satellite-based geospatial reference imagery.  International Journal of Digital Earth. vol. 1, no. 2, pp.221-239.

[2] Budkewitsch, P., Nedelcu, S., Gibson, J., MacGregor, R., Dewing, K. and D. James. 2007.  A Complete Ortho-rectified Landsat-7 Mosaic of Arctic Canada. First International Circumpolar Conference on Geospatial Sciences and Applications, Yellowknife, NT, 21-24 August, CDROM Proceedings, 1p. <BudkewitschP_North2007_A.pdf>