MEMO GIS PROJECTION PARAMETERS USED FOR GAM by dnc16003

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									GAM technical memo 01-01(rev a)
by Roberto Anaya
      Texas Water Development Board, Groundwater Availability Modeling
      (512) 936-2415
      ranaya@twdb.state.tx.us
      February 28, 2001

USING GIS PROJECTION PARAMETERS FOR GAM

The GAM projection parameters are a modified version of the Texas Centric
Projection/Albers (TCP/A) parameters, later renamed as the Texas State Mapping
System/Albers Equal Area (TSMS/AEA). For GAM, the coordinate units are in US
Survey Feet instead of meters and the Latitude of Projection is at 31.25 North instead of
18.0 North.

ESRI did not make their projection algorithms consistent for both ArcView and Arc/Info
and, as a result, extra care must be taken when projecting spatial data into the GAM
coordinate system.

When projecting with Arc/Info, meters must always be used for units of FALSE
EASTING and FALSE NORTHING regardless of whether the MAP UNITS are in units
of feet or meters. When projecting with ArcView or any ESRI products other than
Arc/Info, the units for FALSE EASTING and FALSE NORTHING must be the same as
the units used for MAP UNITS (personal comm. w/ ESRI rep. - Joseph "Brig" Bowles,
jbowles@esri.com).

For Example -

When using ArcView to Project from the View-Properties dialog box, the following
False Easting/Northing and output parameters may be used for GAM standards -

Projection : Albers Equal Area Conic
Units : meters
Datum : NAD83
Spheroid : GRS80
1st Std. Parallel : 27 30 00 (27.50000)
2nd Std. Parallel : 35 00 00 (35.00000)
Central Meridian : -100 00 00 (-100.00000)
Latitude of Projection : 31 15 00 (31.25000)
False Easting : 1500000.00000 (meters)
False Northing : 6000000.00000 (meters)

The MAP UNITS must then be changed from meters to feet from within the View-
Properties dialog box after the projection is complete.


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Alternatively -

When using ArcView Projection Utility, the following False Easting/Northing and output
parameters should be used for GAM standards -

Projection : Albers Equal Area Conic
Units : feet
Datum : NAD83
Spheroid : GRS80
1st Std. Parallel : 27 30 00 (27.50000)
2nd Std. Parallel : 35 00 00 (35.00000)
Central Meridian : -100 00 00 (-100.00000)
Latitude of Projection : 31 15 00 (31.25000)
False Easting : 4921250.15502 4921250.00000 (US survey feet)
False Northing : 19685000.62008 19685000.00000 (US survey feet)

This should be the preferred method for ArcView.

Note: False Easting and False Northing was changed to reflect correction for conversion
error. Any spatial data already projected using previous False Easting and False Northing
will have an insignificant shift towards the southwest of about 0.5 feet. Re-projection is not
necessary. The correct conversion from meters to feet is 1m = 3937 / 1200 US survey ft.
Thanks to Neil Deeds of Duke Engineering for his detection of this error.

Finally -

When using Arc/Info PROJECT command, the following False Easting/Northing and
output parameters should be used for GAM standards -

Projection : Albers Equal Area Conic
Units : feet
Datum : NAD83
Spheroid : GRS80
1st Std. Parallel : 27 30 00 (27.50000)
2nd Std. Parallel : 35 00 00 (35.00000)
Central Meridian : -100 00 00 (-100.00000)
Latitude of Projection : 31 15 00 (31.25000)
False Easting : 1500000.00000 (meters)
False Northing : 6000000.00000 (meters)

This is the only method for Arc/Info users.




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