Sample Distance - Discussion

Profile Extractor can use various type of surfaces - ESRI TIN, Raster or PolygonZ TIN to generate surface profiles for the user defined cross-section line. In general ESRI TIN and PolygonZ TIN are different in the way they store the data, but their behavior is the same so we will refer to both surfaces in this topic simply as TIN.

A profile represents the Z values of a surface along a cross-section polyline using points on the polyline for which the Z will be interpolated from the surface. There are 2 groups of points:

• Vertices of the cross-section polyline - the Z values of the vertices are always interpolated
• Additional data points between the vertices - these are defined by user using the sample distance assigned.

Sample Distance is a regular interval (in the units of the spatial reference of the data frame) at which data points are interpolated from the surface.

When extracting profiles from a Raster surface we need to specify sample distance in order to define the number of data points to be interpolated. The default sample distance used by the Profile Extractor is equal to the cell size of the raster. If more than one raster datasets are used, one of them will be honored in calculation the sample distance.

When extracting profiles from TIN surfaces, the sample distance is not important. Each triangle of a TIN is a plain and needs only 2 data points (the intersection of the cross-section polyline with the edges of the triangle) to be sufficiently represented in a profile.

With Profile Extractor, the user can use sample distance to extract profiles from TIN surfaces. This however in many cases will lead to losing some significant data points in the profile.

The image above shows 2 profiles extracted from a TIN surface for the same cross-section line. The profile in red is extracted without using sample distance. The data points are extracted for the significant points of the TIN (intersections with the edges of triangles). The profile in blue is extracted using sample distance. One can see that some significant points (peaks and valleys) are missing from the blue profile.

When do we need to use sample distance for profiles extracted from a TIN surface.

• If we want to compare profiles derived from different surfaces. In such a case the data points for the profile from each surface need to be the same.
• If we want the data points to be equally spaced along the cross-section polyline.