Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual User Manual

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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.

• 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 folder 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

http://www.ngs.noaa.gov/TOOLS/Htdp/Using_HTDP.pdf