Pima County DOT - City of Tucson DOT

Geodetic Control Points

Support Information and Glossary

Coverage Projection NAD83 to NAD27

HPGPS Coverage Creation

The High Precision Global Positioning System coverage (HPGPS) was created by importing GEOCON_ALL as a text file into ArcInfo using the Northing and Easting (NAD83) coordinates.

NAD27 Coverage Projection

Datums used in the projection of GPS, Geodetic Control Points to Pima County GIS:

The Pima County and City of Tucson GPS, Geodetic Control points are based on a High Accuracy Reference Network (HARN), or High Precision GPS Network (HPGN). These can be thought of as a higher order adjustment of NAD83.
Prior to December, 2000, Pima County GIS data was based on state plane coordinates (NAD27) from ADOT, State Lands Department, and Digitized USGS Quad sheets. NAD27 was the reference datum used by Pima County GIS community. The central Pima County GIS Library was converted to NAD83 HPGN in December, 2000 to match the Geodetic Control Points and corresponding orthphotography.

Users with NAD27 data need to reproject their data to use it with NAD83 HPGN data.

Conversion units used, from US Foot to International Foot

The ArcInfo PROJECT command uses the U.S. Survey foot by default, (one meter = 3.280833333, feet).

The adjustment of GPS Geodetic Control Points was in International feet, (one meter = 3.280839895, feet).

ArcInfo Zone values Zone FipsZone

Arizona, Central AZ_C 3176 202

Projection Methods Used for the GPS Geodetic Control Stations

ArcInfo projection text file, which places the HARN Geodetic Control Points on the existing NAD27 coverages:

/* Projection for COT\Pima GPS\OrthoPhoto\ Stations to NAD27 GIS coverage
/* Source: GPS data: GEOCON_ALL.xls) to -> SQL to ARC/INFO
/* Coverages: input HPGPS, output NAD27    
/* 6/20/99; JLA
/* 01/31/2007 - Change HPGN from zone 3176 to fipszone 0202
/* USE HPGN  
input
projection stateplane
datum HPGN
units 3.280839895
fipszone 0202
parameters
/*to NAD27   
/* 
output
projection stateplane
datum NAD27
units feet
zone 3176
parameters
end

Projecting NAD27 GIS Coverages to HPGPS (The High Precision GPS Network)

ArcInfo projection text file, which places the existing NAD27 coverages on HPGN GPS stations projection.

/* Projection for Pima County Coverages to HPGN GPS position coverage.
/* Source existing county coverage
/* 6/20/99; JLA
/* 01/31/2007 - Change HPGN from zone 3176 to fipszone 0202
/* Use NAD27
input
projection stateplane
datum NAD27
units feet
zone 3176
parameters
/*USE HPGN
output
projection stateplane
datum HPGN
units 3.280839895
fipszone 0202
parameters
end