In May 2003 we delivered our photogrammetric products to a local civil engineering firm.

The data was used in a drainage design for a subdivision.  The project was approximately 900 acres of rural property, mostly covered with heavy brush.  The 1"=250' scale photography consisted of 4 parallel flight lines of 10 exposures each with a pair of control points configured every other model. 

Since a complete surface including areas obscured by vegetation was required, the client supplemented the photogrammetric surface with approximately 500 GPS observed ground shots.  In addition to the GPS data in the obscured areas, the client collected well-distributed points throughout the area as a field check of our mapping.

Before importing the GPS ground shots, we compared our readings to the field data by deriving elevations from the photogrammetric surface at the same horizontal locations of the field shots.  The vertical differences were recorded and the root mean square error (RMSE) was computed to a 99% confidence level.  The results were as follows.


    AREA TYPE                    NUMBER OF POINTS                           RMSE

    OBSCURED AREA                      314                                     0.9' - Unshifted

    BRUSH & GRASS                       184                                     0.3' - Unshifted 

Officially, mapping standards would allow a slight horizontal shift of our photogrammetric readings resulting in even higher accuracy. However, a shift was not applied in order to minimize the time spent gathering the readings.  

Better results can also be expected from readings on 'well-defined' surfaces such as concrete or asphalt, or using a larger scale of photography.

This project was not designed to showcase the accuracy of our work.  Instead, it was a typical deliverable from our operation to a satisfied client. 

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