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FAQ: Why does the Topo to Raster tool generate inaccurate elevation values?

Question

Why does the Topo to Raster tool generate inaccurate elevation values?

Answer

The Topo to Raster tool interpolates a hydrological raster surface from polyline or point data. In some instances, discrepancies occur in the output elevation values. There are multiple factors affecting the result of the Topo to Raster tool such as the nature and dispersion of the input feature data, the use of the drainage enforcement option, and the z-value limit for the interpolation.

• The nature of the input feature data
The tool's algorithm is modified from the Spline interpolation to follow abrupt elevation changes in terrain such as streams, ridges, and cliffs. When the interpolation algorithm is used on flat surfaces with small elevation differences between the contours or the points, inaccurate elevation values are returned in the resulting raster. Depending on the characteristics of the input data, other interpolation methods from the Raster Interpolation toolset, such as inverse distance weighted (IDW), Natural Neighbor, and Spline, can be used. For more information on the differences between these interpolation methods, refer to the following ArcGIS Help document: Comparing interpolation methods.

• The dispersion of the input feature data
Data with well dispersed elevation points yields better results than heavily clustered data, because the Topo to Raster tool produces raster cells based on the commonly occurring surrounding elevation values.

• The output cell size
The default output of the tool is either the width or the height of the extent (whichever is shorter), divided by 250. However, setting a smaller output size yields a raster, which is better at including small variations in elevations across the extent.

• The use of the Drainage enforcement option
The Drainage enforcement option in the Topo to Raster window can be set to fill all sinks, fill unidentified sinks for input data with defined sinks, or fill none of the sinks. In some instances, filling the sinks generates inaccurate elevation values in the resulting raster. Select the NO_ENFORCE option in the drainage enforcement field.

• The use of smallest and largest z-value
The optional largest and smallest z-value can be set in the Topo to Raster window to limit the interpolation, thus minimizing error.

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