4. Datum-to-Datum Grids

A conversion is within a datum, between datums is a transfromation.

To go from grid coordinates on one datum to grid coordinates on another can be done one of two ways.

a. Through geodetic positions

Figure N-4 shows the process which combines two conversions and one transformation.

Figure N-4 UTM NAD27 to SPC NAD83(1986)

This method has three steps:

1. Convert UTM coordinates to Geodetic on NAD27
2. Transform NAD27 Geodetic coordinates to NAD83 Geodetic
3. Convert NAD83 Geodetic coordinates to SPC

Steps 1 and 3 do not introduce position error, step 2 does. 

b. Direct Grid-to-Grid

We could build a model to perform a transformation directly between both grid coordinates systems, Figure N-5.

Figure N-5 Direct Grid-to-Grid

Because this is a 2D situation, to build a model requires at lease two points whose positions are known in both grid systems. Three or more allow a least squares solution which can indicate expected uncertainties in the transformed positions.

Model criteria is left entirely to the user. Using transformation software enforcing various conditions allow investigation of differnt options from which the "best" can be selected for the situation.