Creating a Best-Fit Coordinate System

To relate a local coordinate system to a projected or geodetic system, you must first create a Coordinate System Reference file (*.csr). This file contains a matched list of points in a known system and in the a Geodetic system, and can be created using the Point Database Conversion tab of the Geographic Calculator. The first step is to load your data into the point database grid. This can either be done by loading a data file containing the points (in xls, dbf, SEGP1, or any custom defined text scheme,) or by adding values to the grid manually. The grid needs to contain points from your local coordinate system, and points in a known coordinate system. The Input Coordinate System frame should be set to the coordinate system of the known coordinates. The output Coordinate systems should always be set to Geodetic Latitude/Longitude, using WGS84 as the datum. (Note: To complete the Best-Fit, the format of degrees for output must be set to signed Decimal Degrees.)

Once all of the data is loaded, click on the Conversion Settings button beneath the data grid. This should display the Conversion Settings Dialog.

Input Panel

Original North / Original East - These values should point to the column containing the northing and easting of your local coordinate system.
Transform North/ Transform East - These values should point to the column containing the northing and easting of the known coordinate system.
Point Use Flag - This is a boolean value that determines if the point is used in the Best Fit transformation (any non-zero value), or is only a reference point. (If this column does not exist, a new column can be added to the grid with the Add button.)
Point Name - This is the column containing names of the reference points (optional).

Output Panel

Latitude/Longitude - These values should point to the columns containing the WGS84 Lat/Long output values. (If this column does not exist, a new column can be added to the grid with the Add button.)
North Error / East Error / Lat Error / Long error - These values point to the columns containing errors associated with the Best Fit transformation. They will be displayed in the error graph.
Flag Errors Greater Than - This value is used to highlight errors in the Best Fit graph.

Once all the necessary columns are set in the Conversion Settings Dialog, click OK to close the dialog. Next click the Best-Fit button to complete the Best-Fit. Clicking on the Display Error Plot button (or selecting Window>Best-Fit Error Plot from the drop-down menu) will display a graph showing residual errors created in the Best Fit transformation process.

To save the CSR file, click the Save Data button, and save the file to a known location. This is the file that will be used to create your local coordinate system. It is often best to save the CSR file to the data directory, under the Geographic Calculator installation folder, since this is where the rest of the files relating to coordinate system defintions are installed.