GPS TrackMaker Reference Manual page 118

These same points if converted for UTM system, will present different azimuths, because they will be relative to
the cartographic UTM grid. Besides, they will present cartographic distances:
Northing
Vertex 1 9612901.869
Vertex 2 9613005.503
Vertex 3 9668291.453
Vertex 4 9668202.566
Vertex 1 9612901.869
After the topographical conversion, the angles will be adjusted for the true north and the distances will be adjusted
for the local topographical surface. The table below shows the new values after the topographical conversion:
Point 1 9612901.869
Point 2 9612887.072
Point 3 9668179.140
Point 4 9668192.888
Point 1 9612901.869
The parameters were:
Datum: SAD 69 Average
-
Point of Origin: Point 1
-
Average Altitude : 200 meters
-
Note that the azimuth of the segment that contains the origin point (Point 1) have the same value of the azimuth
according to the true north. The other azimuths were close to the original values, but they were not equal due to
the change of the cartographic lengths to topographical in relation to the origin limit.
The coordinates obtained after the conversion are not cartographic, therefore, they cannot be sent back to GPS
anymore. A way of improving the data presentation would be to choose the Point 1 as origin, subtracting the
values of the coordinates of the Point 1 from the other points. This process could be done outside of the GPS
TrackMaker®, through some electronic table sheet program (ex: Microsoft Excel®):
Vertex 1 0 m
Vertex 2 -15 m
Vertex 3 55277 m
Vertex 4 55287 m
Vertex 5 0 m
NOTE: The file with this practical example is called area_topo.gtm and it is on the GTM PRO® CD and in the
home-page of the program:
Warnings:
Sub metric accuracy will be obtained only by topographical equipment like Total Station or Differential GPS that
cost much more than a handheld GPS. So, for works that demand accuracy, handheld GPS units must only be used
as a support tool and not as the main instrument.
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 of your region.
Easting
277816.512
333377.182
333295.107
277707.029
277816.512
277816.513
333375.263
333403.647
277816.506
277816.513
0 m
55558 m
55587 m
0 m
0 m
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Azimuth (North of Grid)
89.89º
359.91
269.91
179.89
90.02º
0.03º
270.01º
180.00º
118
Datum Table. Use the specific