User Manual Owner's manual

GPS Overview
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apples and oranges. For example, if you set your base on a point with
a known coordinate in the NAD83(1996.0) datum, then all the rover
positions will be in the NAD83(1996.0) datum. Similarly, if you set
the base on an autonomous position, and occupy one or more NGS
control monuments all in the NAD83(1999.0) datum, then the
localization corrects your entire survey to the NAD83(1999.0) datum.
If you need to mix control coordinates from different epochs of the
NAD83 datum, such as using an OPUS solution in the
NAD83(2002.0) datum and an NGS control sheet location in the
NAD83(1999.0) datum, you must transform both coordinates into the
same epoch of the datum. This can be accomplished with the NGS
software for Horizontal Time Dependant Positioning (HTDP).
Richard Snay of the NGS has written a good paper with a description
of the theory behind how plate tectonics affects survey control, and
how the HTDP software can be used to transform locations in North
America into the same epoch of time. This paper can be found on the
NGS web site.
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• High Accuracy Reference Network (HARN)
In the United States, the bulk of the measurements used to establish
NAD83 were conventional. These measurements contain slight
systematic errors that conflict with GPS measurements, which are
more precise over long distances. To address this problem in the
U.S.A., in 1988 the National Geodetic Survey (NGS) began to update
NAD83 coordinate datums with HARN GPS surveys on a state-by-
state basis. These HARN surveys determined small (< 5_cm)
corrections to the location of A and B order control monuments across
the states.
Survey Pro performs a grid transformation for HARN networks in the
United States using the NADCON datum files in *.DGF format.
Note: To use a grid datum, you must have the pair of *.dgf files for
latitude and longitude shift in the Geodata directory on the data
collector.
• Custom Datum Transformations
Most North American and international datums are pre programmed
into the Survey Pro coordinate system database. If you require a
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http://www.ngs.noaa.gov/TOOLS/Htdp/Using_HTDP.pdf