Premise:

Setting the magnetic declination on a compass is a critical step in accurate measurement of bearings that can be translated to map or survey. As the location of the magnetic north pole is constantly moving, the magnetic declination reported on older topographic maps or compass housings are probably out of date. Current declination values for a single point, or for large regions can be found on the NOAA sites:

• single point
• global maps

These resources are helpful, but a current, detailed (state-level) map of magnetic declination would be interesting.

 
Premise:
A generic press-quality plotting system for GRASS, based on GMT. Still a ways to go.

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Two visualizations of the deviance between coordinates positions, at the control point locations.

 
... Continuing from the initial example session in R ...

 
First the output from R:
Looking at the residuals from the regression model used to map bad coordinates (x,y) to good coordinates (nx,ny):

Based on the results of the proof of concept example developed previously, a single function was developed in the procedural language for the PostgreSQL database system called

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The following is a proof of concept query showing how a PostgreSQL query could give the transformation parameters required for an affine transformation.

This is a start for discussions to create a series of function to perform an affine transformation of a PostGIS data set using a table of control points.

 
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