10.4 Calculating Topographical lengths of Tracklogs and Routes
This option is available only in
GPS TrackMaker Professional®
.
The calculation of topographical distance benefits the local
topographical surface, the Tracklog, or the Route to be
measured, making length values more accurate than the value
obtained in the cartographic grid. For further information about
the difference between such surfaces, see
Topographical
Surface x Cartographic Surface
To calculate the topographical length, select the Tracklog or Route
according to the
Data Selection
section and click on the
Topographical
Length button
on Tool Bar or in the
Tools menu.
The window of altitudes will appear, offering two options for the calculation:
•
Tracklogs Altitude
: Selecting this option the calculation will be done
considering the individual altitudes of each Tracklog segment. This option is
useful when the handheld GPS the Tracklogs altitude sends to the computer, or
when the points of Tracklogs have the altitude field filled in.
•
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 medium
altitude will always be indicated in this field, based on the altitudes of the selected Tracklog.
Remember
: The topographical length calculation will depend on the correct indication of the local altitude. The
altitude is the altitude provided by the GPS, or that is to say, in relation to the Earth’s ellipsoid. Use the specific
datum of your area.
The resulting calculation will be indicated in the
status bar in the lower part of the screen, in the respective
Area
Unit
, as well as in the Report window.
The calculated length can be indicated on the screen as a Waypoint. When creating it, just click on the text box of
the Waypoint
comments, and with the arrow keys, choose the value. When the length and the area are calculated,
the values are registered hidden in the text box of the Waypoint
comments.
Warnings
: Sub metric accuracy is only obtained with topographical equipment like Total Station or Differential
GPS that cost much more than a small handheld GPS. So, for tasks that demand accuracy, the handheld GPS must
be used only as a support tool and not as main equipment.
Many handheld GPS don't transfer altitudes (relative and absolute) of each point to the program. So, the distance
calculation cannot consider the natural ground elevations. In these cases, the values refer only to the horizontal
distances.
Changes in the datum can produce differences in length calculations. This happens because each datum may have
a proper value from the Earth semi-axis and of its flattening, it can be seen in
Datum Table
. Use the specific
datum of your region.
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