APPENDIX B

Registering Maps to Digitizers

The process of registering a map to a digitizer proceeds as follows:

The user is prompted for map parameters. Refer to the tiepts or mappts user's guides for a description of the user interface and the type of data to be entered.

• The user sparks points to register the map to the digitizer. The number of points used depends upon the map being used. The map coordinates entered by the user are fit to the corresponding digitizer coordinates for a given point. If six or fewer points are sparked, a first order least squares fit is used. Otherwise, a second order least squares fit is used. If the four map corners are not square (i.e., the digitizer to map fit cannot be determined with a rotation, X and Y translation, and X and Y scaling), more than four points are required. Either latitude/longitude or projection (X,Y) coordinates may be used in defining the map coordinates. The user should also keep in mind errors due to map shrinkage and expansion if paper maps are utilized.

• A verification point is sparked. This point is not used in the digitizer-to-map fit calculation and gives an added indication of map registration accuracy. The verification point is a known coordinate marked on the map--a tick mark or some other point whose coordinate is known.

The digitizer is connected between the user's terminal and the host computer system (as shown below).

With this configuration, the digitizer coordinates usually echo on the user's terminal. Currently, ALTEC and COMPLOT 7000 digitizers are supported. This interface should also work with a variety of other digitizers.

The routines which interface with the digitizer expect the following format:

          B XXXXX YYYYY


   where

     B     is the digitizer button pushed,

     XXXXX is the X digitizer coordinate,

     YYYYY is the Y digitizer coordinate,

     <CR> is a carriage return (one character),

           and a blank is a space.

The button numbers for various digitizer functions are described in the digitizer.h include file. This file also contains the digitizer format. The active button numbers may be changed in the include file and become valid when the modules which interface to the digitizer are rebuilt. The digitizer format is not as easily changeable at this time.