| Asian GPS ---> Proceeding ---> 2002---> GPS and Remote Sensing |
GPS for geometric correction of remotely sensed imagery: possibilities after termination of SA
S. K. Katiyar
Ph.D. student
Civil Engg. Department
I. I. T. Kanpur- 208 016
Email skatiyar@iitk.ac.in
Tel. No. 0512-597937
Onkar Dikshit
Associate Professor
Civil Engg. Department
I. I. T. Kanpur- 208 016
Email onkar@iitk.ac.in
Tel. No. 0512-597937
Krishna Kumar
Professor
Aerospace Engg. Department
I. I. T. Kanpur- 208 016
Email kkumar@iitk.ac.in
Tel. No. 0512-597060
S. K. Katiyar
Ph.D. student
Civil Engg. Department
I. I. T. Kanpur- 208 016
Email skatiyar@iitk.ac.in
Tel. No. 0512-597937
Onkar Dikshit
Associate Professor
Civil Engg. Department
I. I. T. Kanpur- 208 016
Email onkar@iitk.ac.in
Tel. No. 0512-597937
Krishna Kumar
Professor
Aerospace Engg. Department
I. I. T. Kanpur- 208 016
Email kkumar@iitk.ac.in
Tel. No. 0512-597060
Abstract
Precise and reliable information extraction from remotely sensed data requires higher accuracy geometric correction. In the pre-processing of high-resolution satellite imagery like SPOT, IRS-1C/D and IKONOS geometric accuracy requirements are more demanding. Geometric correction process utilizes some GCPs for the removal of image distortions, and the correction accuracy depends on the accuracy of GCPs used. In the developing countries like India, the large-scale maps are not available and getting a high accuracy GCP is a big problem. GPS is an alternative tool for the determination of precise GCP coordinates. The differential mode observations are more accurate and reliable as compared to stand-alone mode. In higher accuracy requirements (sub-meter/centimeter level), the dual frequency (L1 and L2) receivers are used in differential mode, which are very costly. The hand-held GPS receivers are very cost effective and after termination of selective availability (SA) of GPS signals, accuracy of these receivers has been improved considerably.
This research paper investigates the accuracy of hand held L1 frequency GPS receiver (GS5), for its suitability in deriving GCPs for the geometric correction of IRS-1C/D, PAN sensor imagery. In order to verify the accuracy of GPS receiver, eight different stations were established in a localized area of about 1.5 ´ 1.5 km. The true distances between these stations were measured with the help of total station, and GPS measured distances were computed from the mean values of repeated observations taken for a period of about one month. At one of the above stations, observations were also taken with the help of dual frequency receiver (SR530). The difference between GPS computed and total station measured distances is about 1 to 4 meter. The coordinates observed by hand-held receiver are deviating from the corresponding values computed from differential GPS by about 5 to 7 meter. The GCP-based geometric correction of one PAN imagery, using hand-held GPS receiver derived GCPs, has yielded sub-pixel RMS error. In absence of SA, the hand held GPS could be a cost effective solution for the determination of GCP coordinates.
Introduction
Satellite images are vital tool in various applications like land use and land cover mapping, Geographic Information System (GIS) etc. Remotely sensed images are often considered as maps of the radiometric properties of the earth's surface. However, these are not usually map-like. A variety of factors involved in the process of image acquisition, introduce geometric distortions, which are removed by pre-processing of the digital imagery. Geometric correction is the process of rectification of geometric errors introduced in the imagery during process of its acquisition.
A major concern in remote sensing information extraction and data handling is to ensure proper geometric properties of the acquired image. The objective of geometric correction of remotely sensed data is to make the image features geographically correct. Methods chosen for geometric correction rely on the purpose of applications, based on geometric precision required. Precision geometrically corrected products are basic necessity for mapping smaller features in the urban area and use of high resolution satellite data like SPOT, IRS-1C/D and IKONOS. In the geometric correction process, precise ground control points (GCPs) are required and accuracy of geometric correction depends on the precision of GCP coordinates.
As technology progresses, the raster picture element (pixel) resolution of digitally acquired remotely sensed imagery continue to become finer in spatial resolution. This dictates the need of ground control points (GCPs) with increased spatial accuracy to geometrically correct remotely sensed data. Conventionally GCPs are derived from maps, but in the developing countries most of the existing maps and medium scale maps are out of date due to large population growth, rapid changes in urban areas and economic development. In order to achieve the sub-pixel level accuracy in the geometric correction of high spatial resolution imagery required scale maps might not be available. In such situations, Global Positioning System (GPS) can prove an alternative tool, which can provide even sub-meter accuracy GCPs.
The use of GPS derived GCPs in geometric correction of satellite imagery, is very common. In the literature, various researchers have reported the use of GPS derived coordinates in the geometric correction of remotely sensed satellite imagery. GCPs were derived for the geometric correction of Landsat TM imagery of Ituri rain forest of northeastern Zaire, by using Magnavox 4400 GPS receiver (Wilkie, 1990). The average precision hand held GPS receiver (Magellan NAV 1000 PRO) was used in differential mode mode, for the geometric correction of SPOT imagery, with accuracy sufficient to meet planimetric accuracies for 1:50,000 scale production from satellite imagery (Clavet et al., 1993). The Landsat TM and SPOT, MSS and PAN images were geometrically corrected with the help of GCPs derived from Trimble Pathfinder Professional GPS receiver (Cook and Pinder, 1996). GCPs were determined by Magellan NAV 5000 PRO GPS receiver in stand-alone mode, for the rectification of Landsat MSS and TM and SPOT images (Kardoulas et al., 1996). Differential global positioning system (DGPS) was used to derive GCPs for the geometric rectification of fine resolution (4m × 4m pixel) aircraft scanner data (Witter et al., 2001).
In the results of above investigations, accuracy of GCP coordinates determined from stand alone single hand held receiver was not very good due to selective availability (SA) of GPS signals. In view of the termination of SA, the accuracy of stand alone hand held GPS receiver require investigations. In order to have cost effective GPS solution, accuracy requirements need to be thoroughly assessed and accordingly decision should be taken on the GPS configuration.
Background
The higher accuracy (sub-meter/centimeter level) GCP coordinate determination with the help of GPS requires DGPS, because of GPS signal degradation from various sources like SA, atmospheric errors etc. After the decision of US department of defense (DOD) on 1st May 2000 to turn off SA of GPS signals, accuracy range of hand held stand-alone GPS receivers has improved considerably as compared to earlier range of about 100 meter (Chalermchon et al., 2001). In this changed scenario, even a hand held GPS receiver in stand-alone mode may also fulfill the medium accuracy requirements, which may be sufficient for the geometric correction of IRS-1C/D imagery. In India, conventional source of GCP coordinates is Survey of India (SOI) topographic maps on scales 1:50,000 and 1:25,000. These maps are based on very old survey work, and for some of the areas even 1:25,000 maps are not available. The GCP accuracy requirement is higher for the geometric correction of IRS-1C/D, PAN images as compared to the accuracy of available large-scale maps.
GPS is available in various configurations depending on the accuracy requirements and accordingly its cost may vary from about 20 thousands to 20 lakhs. The DGPS configuration for higher accuracy is quite expensive and in the developing countries like India cost is one of the major factor in the adaptability of any new technology like GPS. In such circumstances, hand-held GPS may prove to be a cost effective solution when accuracy requirements are not very demanding. At present, different category hand-held GPS receivers are available, depending on the number of GPS signal channels and their frequency. In the geometric correction of remotely sensed data, GCP collection cost can be reduced considerably if we are able to know about achievable accuracy of hand-held GPS.
Objective
The objectives of this research paper are mentioned below:
The present research work was carried out for the Kanpur city and its adjoining area in India. Hand-held GPS receiver of 12 channels and L1 frequency (model GS5 of Leica Geosystems) and DGPS receiver of dual frequency having L1 and L2 channels (model SR530 of Leica Geosystems) were used for the GPS observations. The Trimble make total station equipment was used for the measurement of distances between GPS observation stations, in order to verify the distances computed from GPS observations. The IRS-1C/D, PAN sensor images as per details given in table 1, were used for the geometric correction in ILWIS 3.0 software.
Methodology
Research work was carried out as per the steps mentioned below:
The results of present work are summarized below:
Fig. 1 Spatial distribution of selected GPS observation stations.
Fig. 2 Deviation of GPS observed coordinates: eastings, northings and height above ellipsoid (H) at station no.1 for observations taken on different dates.
Table 1 Details of remotely sensed imagery used for geometric correction.
Fig. 3 Standard deviation of GPS observed coordinates: eastings, northings and height above ellipsoid (H) at different stations.
Fig. 4 Difference in GPS measured and total station measured distances between different stations.
Table 3 RMS error and number of GCPs used in the geometric correction of IRS-1D, PAN imagery
Table 2 Distances measured by GPS and total station, between IITK stations.
Table 4 Error at test GCPs in terms of image pixels.
Conclusion
After termination of SA of GPS siganls, the hand-held GPS accuracy has improved considerably. Geometric correction of one sub-scene of PAN imagery, using GCPs derived by hand-held GPS receiver in stand alone mode, has yielded sub-pixel level geometric correction accuracy. In absence of precise GCP coordinates, a hand-held GPS can prove a cost effective solution for the geometric correction of satellite imagery and other applications.
Table 5 Hand-held GPS observations at stations no.1
Table 6 Mean values of coordinates observed by single and dual frequency GPS receivers at station no.8.
Limitations
Investigations of this research paper are limited for the plannimetric accuracy only, and height information is not analyzed. GS5 receiver accuracy was tested for small area, however investigations are in progress for the testing of its accuracy on precisely established stations in Kanpur, Lucknow and Bhopal cities, using DGPS receivers.
Acknowledgement
This activity is funded by ISRO-IITK cell, I. I. T. Kanpur. The authors wish to thank Shri N. K. Agarwal, retd. Director Survey Training Institute, Hyderabad for useful discussions and suggestions and PG students of Geoinformatics Division, Civil Engg. Deptt., I. I. T. Kanpur for their help in field work.
Disclaimer
The trade and product names are mentioned only for completeness of this paper, and no endorsement is intended by the authors or the institution concerned.
References
Precise and reliable information extraction from remotely sensed data requires higher accuracy geometric correction. In the pre-processing of high-resolution satellite imagery like SPOT, IRS-1C/D and IKONOS geometric accuracy requirements are more demanding. Geometric correction process utilizes some GCPs for the removal of image distortions, and the correction accuracy depends on the accuracy of GCPs used. In the developing countries like India, the large-scale maps are not available and getting a high accuracy GCP is a big problem. GPS is an alternative tool for the determination of precise GCP coordinates. The differential mode observations are more accurate and reliable as compared to stand-alone mode. In higher accuracy requirements (sub-meter/centimeter level), the dual frequency (L1 and L2) receivers are used in differential mode, which are very costly. The hand-held GPS receivers are very cost effective and after termination of selective availability (SA) of GPS signals, accuracy of these receivers has been improved considerably.
This research paper investigates the accuracy of hand held L1 frequency GPS receiver (GS5), for its suitability in deriving GCPs for the geometric correction of IRS-1C/D, PAN sensor imagery. In order to verify the accuracy of GPS receiver, eight different stations were established in a localized area of about 1.5 ´ 1.5 km. The true distances between these stations were measured with the help of total station, and GPS measured distances were computed from the mean values of repeated observations taken for a period of about one month. At one of the above stations, observations were also taken with the help of dual frequency receiver (SR530). The difference between GPS computed and total station measured distances is about 1 to 4 meter. The coordinates observed by hand-held receiver are deviating from the corresponding values computed from differential GPS by about 5 to 7 meter. The GCP-based geometric correction of one PAN imagery, using hand-held GPS receiver derived GCPs, has yielded sub-pixel RMS error. In absence of SA, the hand held GPS could be a cost effective solution for the determination of GCP coordinates.
Introduction
Satellite images are vital tool in various applications like land use and land cover mapping, Geographic Information System (GIS) etc. Remotely sensed images are often considered as maps of the radiometric properties of the earth's surface. However, these are not usually map-like. A variety of factors involved in the process of image acquisition, introduce geometric distortions, which are removed by pre-processing of the digital imagery. Geometric correction is the process of rectification of geometric errors introduced in the imagery during process of its acquisition.
A major concern in remote sensing information extraction and data handling is to ensure proper geometric properties of the acquired image. The objective of geometric correction of remotely sensed data is to make the image features geographically correct. Methods chosen for geometric correction rely on the purpose of applications, based on geometric precision required. Precision geometrically corrected products are basic necessity for mapping smaller features in the urban area and use of high resolution satellite data like SPOT, IRS-1C/D and IKONOS. In the geometric correction process, precise ground control points (GCPs) are required and accuracy of geometric correction depends on the precision of GCP coordinates.
As technology progresses, the raster picture element (pixel) resolution of digitally acquired remotely sensed imagery continue to become finer in spatial resolution. This dictates the need of ground control points (GCPs) with increased spatial accuracy to geometrically correct remotely sensed data. Conventionally GCPs are derived from maps, but in the developing countries most of the existing maps and medium scale maps are out of date due to large population growth, rapid changes in urban areas and economic development. In order to achieve the sub-pixel level accuracy in the geometric correction of high spatial resolution imagery required scale maps might not be available. In such situations, Global Positioning System (GPS) can prove an alternative tool, which can provide even sub-meter accuracy GCPs.
The use of GPS derived GCPs in geometric correction of satellite imagery, is very common. In the literature, various researchers have reported the use of GPS derived coordinates in the geometric correction of remotely sensed satellite imagery. GCPs were derived for the geometric correction of Landsat TM imagery of Ituri rain forest of northeastern Zaire, by using Magnavox 4400 GPS receiver (Wilkie, 1990). The average precision hand held GPS receiver (Magellan NAV 1000 PRO) was used in differential mode mode, for the geometric correction of SPOT imagery, with accuracy sufficient to meet planimetric accuracies for 1:50,000 scale production from satellite imagery (Clavet et al., 1993). The Landsat TM and SPOT, MSS and PAN images were geometrically corrected with the help of GCPs derived from Trimble Pathfinder Professional GPS receiver (Cook and Pinder, 1996). GCPs were determined by Magellan NAV 5000 PRO GPS receiver in stand-alone mode, for the rectification of Landsat MSS and TM and SPOT images (Kardoulas et al., 1996). Differential global positioning system (DGPS) was used to derive GCPs for the geometric rectification of fine resolution (4m × 4m pixel) aircraft scanner data (Witter et al., 2001).
In the results of above investigations, accuracy of GCP coordinates determined from stand alone single hand held receiver was not very good due to selective availability (SA) of GPS signals. In view of the termination of SA, the accuracy of stand alone hand held GPS receiver require investigations. In order to have cost effective GPS solution, accuracy requirements need to be thoroughly assessed and accordingly decision should be taken on the GPS configuration.
Background
The higher accuracy (sub-meter/centimeter level) GCP coordinate determination with the help of GPS requires DGPS, because of GPS signal degradation from various sources like SA, atmospheric errors etc. After the decision of US department of defense (DOD) on 1st May 2000 to turn off SA of GPS signals, accuracy range of hand held stand-alone GPS receivers has improved considerably as compared to earlier range of about 100 meter (Chalermchon et al., 2001). In this changed scenario, even a hand held GPS receiver in stand-alone mode may also fulfill the medium accuracy requirements, which may be sufficient for the geometric correction of IRS-1C/D imagery. In India, conventional source of GCP coordinates is Survey of India (SOI) topographic maps on scales 1:50,000 and 1:25,000. These maps are based on very old survey work, and for some of the areas even 1:25,000 maps are not available. The GCP accuracy requirement is higher for the geometric correction of IRS-1C/D, PAN images as compared to the accuracy of available large-scale maps.
GPS is available in various configurations depending on the accuracy requirements and accordingly its cost may vary from about 20 thousands to 20 lakhs. The DGPS configuration for higher accuracy is quite expensive and in the developing countries like India cost is one of the major factor in the adaptability of any new technology like GPS. In such circumstances, hand-held GPS may prove to be a cost effective solution when accuracy requirements are not very demanding. At present, different category hand-held GPS receivers are available, depending on the number of GPS signal channels and their frequency. In the geometric correction of remotely sensed data, GCP collection cost can be reduced considerably if we are able to know about achievable accuracy of hand-held GPS.
Objective
The objectives of this research paper are mentioned below:
- Accuracy analysis of GCP coordinates observed by hand-held GPS in stand alone mode and its comparisons with the total station measurements.
- Investigation of geometric correction accuracy of IRS-1C/D PAN sensor imagery, using GCPs derived from hand-held GPS in stand-alone mode.
The present research work was carried out for the Kanpur city and its adjoining area in India. Hand-held GPS receiver of 12 channels and L1 frequency (model GS5 of Leica Geosystems) and DGPS receiver of dual frequency having L1 and L2 channels (model SR530 of Leica Geosystems) were used for the GPS observations. The Trimble make total station equipment was used for the measurement of distances between GPS observation stations, in order to verify the distances computed from GPS observations. The IRS-1C/D, PAN sensor images as per details given in table 1, were used for the geometric correction in ILWIS 3.0 software.
Methodology
Research work was carried out as per the steps mentioned below:
- Selection of eight different well distributed GPS observation stations in the I.I.T. Kanpur (IITK) campus, which are separated by distance in the range of 200 to 500 m. Out of the above stations, some stations are obstructed by trees (station no. 3, 5 and 7) and one station is completely free from obstructions (station no. 8) as it is on the roof top of high rise building.
- Measurement of true distance and height difference between above stations with the help of total station equipment of millimeter level precision.
- Selection of permanent features like intersections of highways, roads, canals, railway lines as GCPs in the Kanpur city and its adjoining area for the geometric correction of satellite imagery.
- GPS observations on the above mentioned stations and GCPs, with the help of hand-held GPS (GS5 receiver) in stand-alone mode. These observations were taken on WGS 84 ellipsoid in two different coordinate systems namely: geodetic (latitude, longitude and height above ellipsoid) and projected coordinates (eastings, northings) in Universal Transverse Mercator (UTM) projection system. In case of GPS observation stations of IITK campus, repeated observations were taken at different times and dates spread over a period of about one month.
- Three different GPS observations for longer duration (about 8 to 14 hrs.) on station no. 8 of IITK, using DGPS receiver.
- Computation of mean and standard deviations of observations taken on different dates for IITK observation stations.
- Computation of distances between stations from mean values of eastings and northings observed by hand-held GPS.
- GCP-based geometric correction of IRS-1D satellite PAN sensor imagery in ILWIS 3.0 software, with the help of GCPs derived from hand-held GPS in WGS 84 and UTM system. In the image rectification process affine transformation and nearest neighbor resampling methods were used.
- Determination of geometric correction accuracy at test GCPs, by computing discrepancy between coordinates read from rectified images and GPS observed one.
The results of present work are summarized below:
- Spatial distribution of selected GPS observation stations, in terms of eastings and northings are shown in fig. 1.
- Hand-held GPS observations on different dates for one of the IITK area station are shown in table 5. The deviations of different date observations from mean values are shown in fig. 2
- The standard deviations of GPS measurements (eastings, northings and height above ellipsoid) at various stations are shown in fig. 3.
- The distances between different stations were computed from the mean values of coordinates measured by GPS and these are listed in the table 2, along with the total station measured distances. The difference between GPS and total station measured distances were computed and the same is plotted in the fig. 4.
- For station no.8, GPS observations were also taken with the help of dual frequency receiver for three different dates and mean values of coordinates were computed by single point positioning (SPP) of collected data. Also mean values of the same station coordinates were computed from hand-held GPS observations of different dates. These results are given in table 6.
- For GCP-based geometric correction of remotely sensed images, the root mean square (RMS) errors of affine transformation and number of GCPs used is given in table 3.
- Discrepancies between coordinates read from geometrically corrected images and their hand-held GPS observed values, for various distributed test GCPs are given in table 4.
Fig. 1 Spatial distribution of selected GPS observation stations.
Fig. 2 Deviation of GPS observed coordinates: eastings, northings and height above ellipsoid (H) at station no.1 for observations taken on different dates.
Table 1 Details of remotely sensed imagery used for geometric correction.
| Area | Satellite and sensor | Imaging date | Path and row |
| Kanpur | IRS-1D, PAN | 22-04-2002 | 99/53 |
Fig. 3 Standard deviation of GPS observed coordinates: eastings, northings and height above ellipsoid (H) at different stations.
Fig. 4 Difference in GPS measured and total station measured distances between different stations.
Table 3 RMS error and number of GCPs used in the geometric correction of IRS-1D, PAN imagery
| Area | Satellite and sensor | Imaging date | Path and row |
| Kanpur | IRS-1D, PAN | 22-04-2002 | 99/53 |
Table 2 Distances measured by GPS and total station, between IITK stations.
| Distance between stations | Distance measured by total station (m) | Distance computed from GPS observations (m) | Difference between total station and GPS distances (m) |
| 1 and 2 | 594.58 | 598.31 | -3.73 |
| 1 and 3 | 467.45 | 470.28 | -2.83 |
| 1 and 4 | 504.30 | 500.33 | 3.97 |
| 1 and 5 | 200.54 | 200.01 | 0.53 |
| 3 and 6 | 438.85 | 439.36 | -0.51 |
| 3 and 7 | 316.56 | 315.11 | 1.45 |
Table 4 Error at test GCPs in terms of image pixels.
| Test GCP no. | Error in GCP row (pixel) | Error in GCP column (pixel) |
| 1 | -3.08 | 0.67 |
| 2 | 1.02 | 1.01 |
| 3 | -3.83 | 0.89 |
| 4 | 3.79 | 0.54 |
| 5 | -0.63 | -0.86 |
| 6 | 0.51 | -0.22 |
| 7 | -0.40 | -2.24 |
| 8 | 1.66 | 1.68 |
| 9 | -0.23 | -0.66 |
| 10 | -0.50 | 2.27 |
Conclusion
After termination of SA of GPS siganls, the hand-held GPS accuracy has improved considerably. Geometric correction of one sub-scene of PAN imagery, using GCPs derived by hand-held GPS receiver in stand alone mode, has yielded sub-pixel level geometric correction accuracy. In absence of precise GCP coordinates, a hand-held GPS can prove a cost effective solution for the geometric correction of satellite imagery and other applications.
| Date | Time (IST) | Latitude | Longitude H (m) | Eastings (m) | Northings (m) |
| 09/5/02 | 19.45 Hrs. | 26°30'37.542 | 80°14'07.410" 72 | 423811.00 | 2932438.00 |
| 23/5/02 | 09:20 Hrs. | 26°30'37.542 | 80°14'07.290" 80 | 423808.63 | 2932439.68 |
| 24/5/02 | 09:00 Hrs. | 26°30'37.518 | 80°14'07.305" 69 | 423809.13 | 2932438.75 |
| 25/5/02 | 10:05 Hrs. | 26°30'37.419 | 80°14'07.410" 70 | 423809.61 | 2932436.53 |
| 28/5/02 | 10:40 Hrs. | 26°30'37.590 | 80°14'07.320" 71 | 423809.64 | 2932441.70 |
| 29/5/02 | 17:40 Hrs. | 26°30'37.734 | 80°14'07.224" 85 | 423805.51 | 2932445.42 |
| 30/5/02 | 17:30 Hrs. | 26°30'37.656 | 80°14'07.128" 68 | 423804.17 | 2932443.39 |
| 06/6/02 | 17:30 Hrs. | 26°30'37.674 | 80°14'07.698" 77 | 423809.50 | 2932445.58 |
| 10/6/02 | 18:05 Hrs. | 26°30'37.740 | 80°14'07.374" 74 | 423811.16 | 2932445.39 |
Table 6 Mean values of coordinates observed by single and dual frequency GPS receivers at station no.8.
| Observation details | Latitude | Longitude H (m) | Eastings (m) | Northings (m) |
| SR530 coordinates | 26°30'46.396 | 80°13'54.331" 100.01 | 423451.73 | 2932714.04 |
| GS5 coordinates | 26°30'46.200 | 80°13'54.231" 84.22 | 423448.81 | 2932707.97 |
Limitations
Investigations of this research paper are limited for the plannimetric accuracy only, and height information is not analyzed. GS5 receiver accuracy was tested for small area, however investigations are in progress for the testing of its accuracy on precisely established stations in Kanpur, Lucknow and Bhopal cities, using DGPS receivers.
Acknowledgement
This activity is funded by ISRO-IITK cell, I. I. T. Kanpur. The authors wish to thank Shri N. K. Agarwal, retd. Director Survey Training Institute, Hyderabad for useful discussions and suggestions and PG students of Geoinformatics Division, Civil Engg. Deptt., I. I. T. Kanpur for their help in field work.
Disclaimer
The trade and product names are mentioned only for completeness of this paper, and no endorsement is intended by the authors or the institution concerned.
References
- Chalermchon Satirapod, Chris Rizos and Jinling Wang, 2001, GPS Single Point Positioning with SA off: How accurate can we get? Survey Review, 36(282), pp. 380-386.
- Clavet, D., M. Lasserre, and J. Pouliot, 1993, GPS Control for 1:50,000-Scale Topographic Mapping from Satellite Images. Photogrammetric Engineering & Remote Sensing, 59(1), pp. 107-111.
- Cook, A.E., and Pinder, J.E., 1996, Relative Accuracy of Rectifications Using Coordinates Determined from Maps and the Global Positioning System. Photogrammetric Engineering & Remote Sensing, 62(1), pp. 73-77.
- Kardoulas, N.G., A.C. Bird, and Lawan, A.I., 1996, Geometric Correction of SPOT and Landsat Imagery: A Comparison of Map- and GPS- Derived Control Points. Photogrammetric Engineering & Remote Sensing, 62(10), pp. 73-77.
- Wilkie, D.S., 1990, GPS Location Data: An Aid to Satellite Image Analyses of Poorly Mapped Regions. International Journal of Remote Sensing, 11(4), pp. 653-658.
- Witter, J.D. and Lyone, J.G., 2001, Differential GPS Geometric Rectification of Fine-Resolution Aircraft Scanner Data. Journal of Surveying Engineering, 127(2), pp. 52-58.