ASPRS – American Society for Photogrammetry and Remote Sensing (1990)
| Contour Interval (feet) |
Class 1 Vertical Accuracy limiting RMSE (feet) CONTOURS |
Class 1 Vertical Accuracy limiting RMSE (feet) SPOT ELEVATIONS |
| 1 | 0.33 | 0.17 |
| 2 | 0.67 | 0.33 |
| 5 | 1.67 | 0.83 |
For Class 1 maps according to the ASPRS Accuracy Standards, the limiting RMSE is set at one third the contour interval. Spot elevations shall be shown on the map with a limiting RMSE of one-sixth the contour interval or less
Vertical map accuracy is defined by the ASPRS Accuracy Standards (ASPRS Specifications and Standards Committee, 1990) as the RMSE in terms of the project’s elevation datum for well defined points only.
| Map Scale | Class 1 Horizontal Accuracy limiting RMSE (feet) |
| 1″=5′ | 0.05 |
| 1″=10′ | 0.1 |
| 1″=20 | 0.2 |
| 1″=30′ | 0.3 |
| 1″=40′ | 0.4 |
| 1:=50′ | 0.5 |
| 1″=100′ | 1.0 |
| 1″=200′ | 2.0 |
| 1″=400′ | 4.0 |
| 1″=500′ | 5.0 |
| 1″=800′ | 8.0 |
NMAS – National Map Accuracy Standards (1947)
Vertical accuracy. Not more than 10% of the elevations tested shall be in error more than one-half the contour interval.
Spot elevations: 90% of spot elevations must be within 1/4 C.I. and all within 1/2 C.I. (Photogrammetry for Highways Committee, 1968)
| Contour Interval (feet) |
NMAS Vertical Accuracy limiting RMSE (feet) CONTOURS |
| 1 | 0.5 |
| 2 | 1.0 |
| 5 | 2.5 |
Horizontal accuracy. For maps on the publication scales of 1:20,000 or larger, not more than 10% of the points tested shall be in error by more than 1/30 inch.
| Map Scale | NMAS Horizontal Accuracy limiting RMSE (feet) |
| 1″=5′ | 0.16 |
| 1″=10′ | 0.33 |
| 1″=20 | 0.66 |
| 1″=30′ | 1.0 |
| 1″=40′ | 1.3 |
| 1:=50′ | 1.6 |
| 1″=100′ | 3.3 |
| 1″=200′ | 6.6 |
| 1″=400′ | 13.3 |
| 1″=500′ | 16.6 |
| 1″=800′ | 26.6 |
GSD (Ground Sample Distance)
The Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers measured on the ground. The bigger the value of the image GSD, the lower the spatial resolution of the image and the less visible details. The GSD is related to the flight height: the higher the altitude of the flight, the bigger the GSD value. A GSD of 2 cm equals 2 cm on the ground.
Contour Interval
| Contour Interval | GSD |
| 0.5-Foot | ≤ 1.8 cm |
| 1-Foot | ≤ 5 cm |
| 2-Foot | ≤ 5 cm |
