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How to Calibrate the Digitizing Table

To input data in SPRING through the digitizing table it is necessary that the geographic or plane coordinates be very well related to the coordinates of the table. By calibrating the table we establish this good relationship, within the limits of the desired precision.

The table calibration is necessary for the applications that make use of this peripheral, like the image registration via table and the vector edition, be it in thematic, numeric data or cadastral maps.

In this version of the software are defined some formats of communication with the digitizing table;  so it should be verified in the manual of the table  what types of formats it can understand. Some table models like DIGIGRAF, DIGICOM, CALCOMP, SUMMAGRAPHICS, and KURTA have been tested.

It is suggested the reading of the Installation and Configuration Manual for the configuration of the digitizing table.

Access to the digitizing table is allowed whenever the table is correctly configured and connected, and the environment variables have been define from within SPRING. Otherwise, the option Tools - Calibrate table... cannot be accessed.

Initial Procedures

Before beginning the table calibration operation some other procedures should be performed:

1) The user must have four points for calibration on the map. These points should be well distributed over the region that will be digitized, so that the least distortion will be obtained, for the more we are far from a point, the greater the distortions will be. The figure below shows an ideal situation  for the digitization of a rectangular area, where four points are precisely known in the work area. Note that the distance from the points to the limits of the area to be digitized is half the distance between the points.


2) Position the map on the table and fix it with adhesive tape (opt for paper tapes), so that the map is smoothly stretched. Note that the area to be digitized and the calibration points must be within the sensitive area of the table. The figure below shows the placement of a map on a table.


NOTES:

  • See the item "Peripheral Configuration" in the Installation and Configuration Manual  to guarantee the correct operation of the table. It is also recommended to perform the test of communications between the table and the computer . Use the command "cat /dev/<code of the serial port>" in the UNIX version, as described in the item "Procedures to Test the Digitizing Table" in the same manual.

  • The communications test with the table in the Windows version must be done with the program "testemesa.exe" which is found in the same directory where SPRING was installed. See the details in the Installation and Configuration Manual.


 Calibrating the Digitizing Table:

  • Load a Database and a Project (no need to load a project if the calibration aims at the acquisition of control points for image registration through the table or keyboard);
  • Select Tools - Digitizing Calibration.. ;

OBS: The cartographic projection presented in the "Calibration" window is by default the same as the one in the loaded project. In case the data to be digitized are in another projection it will be necessary to inform that to the system. Thus, during the digitizing process the lines and points are automatically remapped to the projection system of the project.

  • In case the projection of the map attached to the table is the same as the one of the loaded project, click on the button Coordinate System to define the calibration in Plane coordinates (meters), or in Geographic coordinates (degree, minutes, seconds);

      NOTE: When selecting geographic coordinates the syntax should be: a letter indicating both (n) or sth (s) for latitudes, east (e) and west (o) for longitude, followed by degrees, minutes and seconds, spaced by a space character.

      Example: Long: o 45 30 00 Lat: s 22 10 00

      When selecting plane coordinates it is enough to type the values in meters.

      Example: X(m): 456000 Y(m): 7455000

  • Click on button Point 1 to insert the coordinates;

      OBS.: The cursor will change into a small clock indicating that  a point on the table should be acquired with button 1 of the table mouse. The coordinates XM and YM of the table will be updated with the coordinates of the point. One must then fill in the fields Long/Lat or X(m)/Y(m), according to the adopted projection, with the map coordinates of this point.

      The same procedure is applied for points 2, 3, and 4. There is no need to follow exactly this sequence. The points could be acquired first and then the coordinates. Make sure that all the points and coordinates were correctly provided.

  • Click on Execute to associate the table coordinates with the map coordinates.

IMPORTANT: When performing the calibration a file is created with the name of the project, and the extension ".ama" (ex.:"minas.ama"). This file keeps the coordinates of the table and map of the four calibration points, related to the loaded project, this way even after closing and then reopening the SPRING module there is no need to repeat the calibration unless the map is repositioned over the table or is replaced by another.

To test the calibration

To verify if the calibration points are precise or acceptable for the work scale you can choose one of the four calibration points or any other, whose coordinates are previously known, to test the calibration.


Testing the Calibration:

  • choose a point in the map whose coordinates are known;
  • click on the button Test and then click on button 1 of the table's mouse on this point;
  • verify if the values presented for the coordinates are approximately the same as the values of the chosen point.
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NOTE: To make sure that the calibration lies within the desired precision remember that the precision of a map is not smaller than 0.3 mm times the denominator of the map scale, and this way the precision of the calibration is limited by this value. In case the calibration is not within the desired precision, repeat the operation acquiring the points again.

To modify the projection of the map on the table

In the case of the projection of the map taped to the table be different than that of the loaded project the user must inform it.

Modifying the projection of the map taped to the table:

  • in the window "Calibration" click on the button Projections... to change the parameters;
  • in the window "Projections" select the System, the Earth Model, among other parameters that depend on the projection used (see more about the operations on this window)
  • click on Execute to put into effect the new parameters of the map that will be digitized.

NOTE: After defining the protection and the coordinate system the introduction of the calibration points follows, a described above.

Calibration without loading a project

In the case of image registration with a map, there is no need to load a project, but the map projection should also be informed so that the control points can be incorporated to the image with the correct projection reference. (see more about image registration).