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Poster Session 3
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Evaluation of Handy GPSs and Their Applications
3. Error by Differential GPS
More accurate positioning is possible by handy GPS using differential method (Tomita and Matsuyama 1996, Tsuyuki 1994). The "Germin GPS-38" has function to receive the Differential GPS (DGPS) data, however it requires spatial services by a GPS company which provide DGPS information by telephone communication and this is not adequate for using field investigation in abroad. In this paper, pseudo-differential method was tried. The method is (1) to receive GPS data at two point simultaneously, one is already known in position and another is unknown point, (2) to calculate the error of received GPS data at known point and (3) to subtract the calculated error from the GPS data of unknown point at the corresponding time. Fig-4, 5 and 6 show the received latitude, longitude and altitude by GPS at two different points where A is known and B is unknown. From the figure, the fluctuation seems synchronized for two points. By calculating error of point-B using pseudo-DGPS method, error evaluation was made.
Fig-4 psedo-differential dat (latitude)
Fig-5 pseudo-differential data(logitude)
Fig-6 pseudo-differential data(altitude)
There still remain error by pseudo DGPS, however the accuracy was very much improved than single GPS method. The 5 minutes averaged error of horizontal distance of pseudo DGPS is 1m to 33m, and it is more stable than single GPS error which is 13m to 69m. The 10 minutes averaged horizontal distance error of pseudo DGPS is 2m to 14m. Table-1 shows the result of error evaluation for various-types of data correction.
Table-1 Result of Error evaluation of various type GPS correction
| |
|
Latitude |
Longitude |
Horizontal Dist. |
Altitude |
| Single GPS |
Raw Data |
-130m/74m |
-38m/52m |
0m/132m |
-289m/71m |
| |
5min Ave. |
-67m/39m |
-16m/31m |
13m/69m |
-235m/51m |
| |
10min Ave. |
-33m/28m. |
-6m/25m |
11m/34m |
-159m/34m |
| |
30min Ave. |
-20m/-2m |
9m/15m |
12m/25m |
-4m/15m |
| Pseudo-DGPS |
Raw Data |
-40m/49m |
-31m/33m |
0.5m/52m |
-66m/46m |
| |
5min Ave. |
-25m/27m |
-14m/18m |
1m/33m |
-28m/34m |
| |
10min Ave. |
-10m/11 m |
-10m/8m |
2m/14m |
-11m/-2m |
4. Required Accuracy for R.S. Data
The error of handy type GPS is a few decades meters to hundred meters by single-type GPS and several meters to a few decades meter by pseudo DGPS. How much accuracy is required for field investigation for R. S. data. Position information is discussed in two ways in this paper. Case-(a) is that a point is used as a ground control point (GCP) and Case-(b) is that a point used as ground truth (GT) data.
- Point as GCP
A pixel in R. S. data is shown as gray color in Fig. 7. Now the GPS receiver is standing in the pixel, but the exact point the pixel, but the exact point in the pixel is not clear, and it may be at point-B in the worst case. When the GPS positioning error is d(m) in horizontal distance, the GPS indicated point -P in Fig. 7. And the maximum error of GCP positioning, that is the distance between-A, which is the exact center of the pixel, and point-P, is 2 n + d(m), where n is the pixel size. If the GCP accuracy requires less than 1 pixel error, the "d" must be less than 0.3 n (m) which is about 9(m) in case of the data is LANDSAT-TM data where the pixel size(n) is 30 m.
Fig-7 Error for gcp
- Point as GT
Remarked pixel is colored gray in Fig-8. The GPS receiver is standing in the pixel, but the exact point in the pixel is not clear. When the positioning error is adopted within 1 pixel, the required GPS accuracy is also within 1 pixel which as shown as dotted line in Fig-8. When LANDSAT-TM data is used, the required GPS error is less than 30m.
Fig-7 Error for GT
The required GPS accuracy is different depending on the purposes.
5.Discussion
When the "Germin GPS-38" which was used in this paper is used for field survey with LANDSAT-TM data, the acquired point data by calculating 10 minutes average in pseudo-DGPS method is not enough as GCP data because the error includes sometimes 14 m which is more than the GCP's required error which is about 9m for TM data . On the other hand, when the position data is used at GT data, 30 minutes averaged data by single GPS or 5 minutes averaged data by pseudo-GPS are acceptable.
Geodetic systems and projections were not considered in this paper. The "Germin GPS-38" supports various geodetic systems rather than WGS-84. Attentions must be paid for field survey in abroad with GPS and local maps.
References
-
Tomita, K and Matsuyama, H, 1996. Development and Investigations of Semi-Differential GPS,
- Theory and Application of GIS, Vol 4. No 2, GIS Association of Japan, pp. 15-27
- Tsuchiya, J and Tsuji, H, 1995. GPS Sokuryo-no Kiso, pp11-170
- Tsuyuki, S, 1994. Sinrin Tyosa ni Okeru GPS riyono Kanosuei, Journal of Japan Forest Planning, pp. 45-58.
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