![]() How to Calibrate the Digitizing TableTo 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:
Calibrating the Digitizing Table:
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.
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
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.
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
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