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 Easting Azimuth (North of Grid)
Vertex 1 9612901.869 277816.512
Vertex 2 9613005.503 333377.182 89.89º
Vertex 3 9668291.453 333295.107 359.91
Vertex 4 9668202.566 277707.029 269.91
Vertex 1 9612901.869 277816.512 179.89
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 277816.513
Point 2 9612887.072 333375.263 90.02º
Point 3 9668179.140 333403.647 0.03º
Point 4 9668192.888 277816.506 270.01º
Point 1 9612901.869 277816.513 180.00º
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 0 m
Vertex 2 -15 m 55558 m
Vertex 3 55277 m 55587 m
Vertex 4 55287 m 0 m
Vertex 5 0 m 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: www.trackmaker.com
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 Table. Use the specific
datum of your region.
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