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ErrorsStream Channel OffsetThe stream channel data downloaded in ArcView format were imported into IDRISI, but the offset between the imported stream channel network and the stream channel network generated within IDRISI was considerable (60-180 m in places). In the image below, white lines are stream lines imported from ArcView, and red lines are the stream channels generated in IDRISI using the DEM.
This is most likely due to shifts in position caused by changing from the NAD 1983 datum to the NAD 1927 datum. Raster to vector conversionsInterestingly, I found that converting from raster to vector (and back) did not introduce any error, because the IDRISI raster-vector conversion is accurrate to the nearest pixel. I had expected this to be a major source of error. Error inherent in the DEMThe metadata for the DEM used in this analysis indicate that the data was produced by scanning USGS topographic maps as vector files and interpolating between the resulting contour lines. The original contour interval in the maps varied between 5 and 40 feet (1.5 to 12 m). The 10-meter DEMs generated from this data capture essentially all of the information in the original contour lines. Unfortunately, generalization and interpolation inevitably introduced some error in the data, but it is reasonable to assume a vertical accuracy of 10 m, with respect to the original maps used to generate the DEM. Since I don't know which original maps were used to generate the DEM, I cannot make a statement about accuracy with respect to the real world. However, a vertical error of 10-20 m is not important when there's a 250-meter high wall of mud and debris bearing down on you. Therefore, I consider the error inherent in the DEM to be acceptable for this analysis. Next: References >> |
