Before map tie-down can occur, it is necessary to establish communication with the tablet or digitizing table. This is done by setting the various serial communication parameters, and then pressing the ``Connect to Tablet'' button described under Tablet Device Setup.
Once this is done it is necessary to tiedown four to five points on the map by pointing to them on the tablet, and typing in the georeferenced location. This is described under Establish Map Tie-down.
If a map tie-down has been performed previously, the values entered on that occasion will be reloaded so that it is only necessary to establish the above parameters once, though the ``Connect to Tablet'' button has to be pressed to establish communication with the tablet every time.
A new tie-down should be performed if the map is changed or moved.
The Device field should contain the name of the serial port to which the tablet is attached. For Unix systems, this is something like /dev/ttys1 or /dev/ttya. For PC systems (Windows, NT and OS/2) this would be COM1 or COM2.
The Baud Rate, Parity, Stop Bits, and Data Bits fields should be set as appropriate for the tablet or digitizing table being used.
Further information on setting tablet communications parameters and trouble shooting tablet communications is available in the TABLET help file.
The Tablet Type should be set to the type of tablet being used. If your tablet is not on the list of supported tablets you can create a new tablet definition. See 'Configuring Other Tablets'.
Once the communication parameters have been set, the ``Connect to Tablet'' button should be pressed to establish contact with the tablet. Then press a button on the tablet to test the communication. The Xpos, YPos, and Button fields should be updated on the screen when the button press is received from the tablet. If it is not, then some aspect of the communication is not working. Review the communication settings, serial device name, tablet type, and wiring to the tablet.
See Also: Tablet Setup and Map Tie-down, Configuring Other Tablets
Before proceeding with map tiedown, it is necessary to have established communication with the tablet, as described in Tablet Device Setup.
Then it is necessary to select the coordinate system in which map coordinates will be entered by selecting from the coordinate system option menu. This is done with the GeoEdit control.
Following this, it is necessary to enter four to five reference points on the tablet. This is done by moving the tablet pointer to a point on the map, and pressing one of the buttons on the pointer. Then enter the X and Y location in the text fields associated with one of the reference points in georeferenced coordinates, and press the ``Not Set'' button beside the point.
Normally the reference points are the corners of a cartographic map and a point in the centre . However, any four to five points may be selected. Using five points to tie down gives a better estimate of the map positioning to GCPWorks.
To modify a tiedown point, it is possible to change the georeferenced coordinates in the text fields at any time before the Tie-down Map to Tablet step is complete. As well, the corresponding tablet position can be changed by moving the tablet pointer to a new position, pressing a button on the tablet pointer, and then pressing the button beside the corresponding reference point.
See Also: GeoEdit
If a .prf file exists, and does indeed contain tiedown information, the user will be prompted to either continue with this information or to start a completely new tiedown (in which case, only the tablet setup information is kept) when the tablet configuration panel opens for the first time. If the user selects to continue, values for the fields on the panel will be filled out from the file. These fields may be reset by the user if need be.
If the user elects to start a new session, then this information will not be used to default the fields. In this case, the user will have to fill out all the fields in the panel before continuing to GCP collection.
Grid pinning helps eliminate errors that creep into a tie-down to a map due to distortions in the material of the map. The grid square is used as a smaller area to correct the pinned GCP.
The grid ``square'' is initially defined by clicking on the ``Define Grid Pin Box...'' button from the Map Tiedown window, which opens the ``Grid Definition Panel''. If the projection has been defined in the GeoEdit area of the Map Tiedown window, Geocoded or Geographic are available. If the projection definition is incomplete, only Geocoded is available. Geocoded is a grid of the map projection eastings and northings and are square or rectangular. Geographic means the grid of LONG/LAT lines transformed to the map projection and may appear non-square or non-rectangular.
To use grid pinning, first pin the GCP, then pin the four corners of the grid square around it. If the GCP is not within a "complete" grid square (this can happen at the edge of the map), then collect four non-linear grid corners close to the GCP. DO NOT collect the edge of the map as a grid square.
The grid surrounding the GCP may be multiples of the initial grid defined. For example, if the initial grid defined was 5 minutes x 10 minutes grid pinning could be done by pinning an area of 30 minutes x 30 minutes with the grid corners being rounded to the nearest 5 minutes Longitude and 10 minutes Latitude. GCPs collected within the same grid pinning area will not require redefining the grid pin window and will reuse the existing grid pinning area.