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Applied Single GPS Point Statistical Analysis to Geometric Correction of Digital Satellite Imagery Field Work The used image for the analysis is collected in the year 2003 for the area around the office of the Department of Water Resources Studies (DWRS) in the city of Abu Dhabi. It is relatively flat and very close to sea level so that errors related to terrain differences can be ignored. Image displacement were recognized but ignored because the test aimed to study errors at worse conditions. A single point above DWRS building were selected for the initial single GPS point statistical analysis and used as a reference for accuracy check. Nine points were selected to measure location data (Figure 1).  Figure 1. The selected image with the ten studies points (courtesy of MAPSGeosystems). All GPS data were collected using latitude/longitude readings in the form of degree, minutes, and decimal seconds (DD, MM, SS.SS). The GPS used were selected so that it is sensitive to the nearest two digits after the decimal second for high accuracy. All latitude/longitude readings were transferred into the metric units of WGS84 UTM zone 40 before any calculations or analysis done. Table 1, shows the summary single GPS statistics for the reference point above the DWRS building. The distance measurements from all readings to the mean center are calculated and summarized in the Column “Distance”. The standard errors as expected are in the decimeter level and estimated at 22 centimeters in the Easting direction and 23 centimeters in the Northing direction. Figure 2, is the scatter graph of the GPS readings for the reference point. Table 1. Summary statistics for the selected reference point.
Note: All measurements are in meters.
 Figure 2. GPS point distribution at the reference point. The same process was applied to the all selected points. Table 3, shows the summary of the results that included location in Latitude/Longitude, UTM Eastings and Northigs, and standard errors are included. Table 3. Summary results for all points Latitude, Longitude, Eastings, Nothings, and Standard Errors.
Note: All measurements are in meters.
The information in table three was used to perform the geometric correction to the original image. During the process, point 9 was found to be a clear blunder. The team was not able to determine why the GPS readings were wrong. The best guess was that a mistake was made during reading the numbers at the seconds’ level (not at the decimal second). The geometric correction program recommended the removal of points 5 and 8. When investigating a possible reason for that, the team suggested that heavy traffic and buildings were the cause of the high RMS values. In other words, the systematic error “multi path” is the probable cause for the error. The team also suggested that another possible reason for the error is caused because of difference in time between the image and analysis date. Table 4, summarizes the geometric correction Errors. Table 4. Summary image geometric correction.
The table suggests a maximum error on the entire image to be 3.54 meters on the Easting direction and -2.3 on the Northing direction. The maximum RMS is 3.59 meters. Figure 3, is an image map that show the mean UTM grid with Latitude/Longitude tic marks. When the reference point checked for accuracy, the point location was about 1.94 meters from where it should be.  Figure 3. An image map for the selected area. |