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How do I create a custom WKT to change meters to feet?

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Jeff
  • Updated

In this example we will change the well-known text (WKT) string of a coordinate reference system (CRS) from meters to U.S. feet.

Copy the WKT string of the CRS you would like to modify. The following example uses Maine West, NAD83, meters.

Code:

PROJCS[“NAD83 / Maine West”,
GEOGCS[“NAD83″,
DATUM[“North_American_Datum_1983″,
SPHEROID[“GRS 1980″,6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
AUTHORITY["EPSG","6269"]],
PRIMEM[“Greenwich”,0,
AUTHORITY["EPSG","8901"]],
UNIT[“degree”,0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4269"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",42.83333333333334],
PARAMETER["central_meridian",-70.16666666666667],
PARAMETER["scale_factor",0.999966667],
PARAMETER["false_easting",900000],
PARAMETER["false_northing",0],
UNIT[“metre”,1,
AUTHORITY["EPSG","9001"]],
AUTHORITY["EPSG","26984"]]

 

First, remove the last two AUTHORITY[] tags that are a part of the UNIT[] parameter; they will no longer apply to the new WKT in US Feet. The end of the WKT should now look like this:

Code:

PARAMETER["false_northing",0],
UNIT["metre",1]]

 

Next, identify the conversion factor and unit name of your preferred unit (US FEET). In this case, the unit name is “Foot_US” and the conversion factor is 0.304800609601219241. Enter these values in the UNIT parameter:

Code:

UNIT["Foot_US",0.304800609601219241]]

 

Divide the values for “false_easting” and “false_northing” by the conversion factor:

900000/0.304800609601219241 = 2952750 (rounded up)
0/0.304800609601219241 = 0

Enter these new values for false easting and northing in the WKT:

Code:

PARAMETER["false_easting",2952750],
PARAMETER["false_northing",0],

 

The new WKT can now be used as a custom WKT in GeoExpress. Here is the completely modified WKT:

Code:

PROJCS[“NAD83 / Maine West”,
GEOGCS[“NAD83″,
DATUM[“North_American_Datum_1983″,
SPHEROID[“GRS 1980″,6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
AUTHORITY["EPSG","6269"]],
PRIMEM[“Greenwich”,0,
AUTHORITY["EPSG","8901"]],
UNIT[“degree”,0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4269"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",42.83333333333334],
PARAMETER["central_meridian",-70.16666666666667],
PARAMETER["scale_factor",0.999966667],
PARAMETER["false_easting",2952750],
PARAMETER["false_northing",0],
UNIT["Foot_US",0.304800609601219241]]

 

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