GPS TrackMaker Reference Manual page 117

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Local Average Altitude: This option is useful when the user wants to use an average altitude for the area,
or when there are no altitude registrations in the Tracklogs. A suggested average altitude will always be
indicated in this field, based on the altitudes of the selected Tracklog.
Warning: The topographical area calculation will depend on the correct indication of the local altitude. The
altitude is the altitude provided by the GPS, that is to say, according to the Earth's ellipsoid. Also, use the specific
datum of your region.
Important: After conversion, the coordinates of the points in UTM system will be converted to the local
topographical surface. Because of that, the new coordinates must not be sent back to the GPS, because they
won't be in the UTM system anymore, but, they will be in the local topographical surface.
They should not be taken as geodesic coordinates anymore, but as local topographical coordinates.
Example
The area #1 will be the subject of the topographical conversion. It has angular dimensions of 0.5 degree (SAD69)
perfectly aligned with the true north, as indicated below:
Latitude
Vertex 1 -3.5
Vertex 2 -3.5
Vertex 3 -3.0
Vertex 4 -3.0
Vertex 1 -3.5
The best way to understand the topographical conversion is with a practical example.
The side illustration shows three-square areas with identical angular dimensions,
located in the extremities and in the center of the UTM zone 22M, in Para State/Brazil.
Longitude
-53.0
-52.5
-52.5
-53.0
-53.0
Azimuth (True North)
90º
0º
270º
180º
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