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      Run `TABLET' 'ENTER' and select the option CAL by typing 'CAL'  'ENTER'.  Click on a known coordinate point on the Tablet and enter the coordinate  value (e.g. 4, 4). Click on a second known point on a diagonal line from the first, and  enter the coordinate value of the point (e.g. 7,7). Press the 'ENTER' key in response to  the request to enter a third point, and then in response to the query 'Select Transformation type' select orthogonal (o). Your tablet will now be calibrated.
    Entering only two calibration points assumes that your drawing is undistorted, e.g. unit distances are the same in both X and Y directions, and all supposed orthogonal lines are really orthogonal.  You can however enter three points for an 'Affine' transformation, or as many as nine points for a 'Projective' transformation, if you think your map is physically distorted. An 'Affine' transformation consists of translation, independent scaling of X and Y, rotation, and skewing, and assumes that all parallel lines really are parallel. In the case of a projective transformation the drawing will be differentially stretched (rubber sheeting). Straight lines map into straight lines but parallel lines may not stay parallel.
     Draw a rectangle on a rectangle layer to represent the  extents of your drawing, and then carry out a 'zoom' 'all'. After an interval of time the  cross hairs should appear on the screen. While you are in Tablet mode, the Menu Bar at  the top of the screen is no longer accessible, and the Tablet cursor no longer functions as  a mouse. In order to access the Menu Bar it is necessary to give the command  'TABLET' `OFF' or use the toggle switch 'CTRL T'. To return to Tablet mode again  use the toggle or the command 'TABLET'  'ON'.  Objects drawn or traced on the Tablet  will be replicated on the screen, with the same coordinates as the object on the Tablet. It may also be possible to toggle between 'Mouse' and 'WYPIWYG' modes by appropriately setting button 2 on the tablet button mouse, or using both the tablet and the normal computer mouse.

 IMPORTANT: if the objects you are going to digitize are on maps of different scales, you will need to re-initialize the digitzing tablet before re-calibrating the new map. Enter 'REINIT' at the command line and click the 'Digitizer' check box under 'Device and File initialization'.

        Sketch some object you have drawn on the sheet of graph paper place on the  surface of your tablet.
        Select LAYER CONTROL and make the the '1granith'  layer CURRENT. In  the DRAW menu select Hatch and select a hatch pattern. Select the object you have drawn and have Autocad draw a Hatch Pattern.

           AutocadDrawing tips:

         Make a layer for each polygonal unit (rock), line (fault, river, road),  point  (outcrop),  or text object.
         Draw all objects which are enclosed by a single boundary.
        Where objects share a common boundary, marks the dominant boundary of  each object 1, 2, 3, etc, progressively outwards towards the rectangle boundary.
        Trace  each boundary onto its corresponding unit layer, extending each line beyond the point  where it crosses any lines already drawn, including the rectangle boundary.  Use Bpoly  to construct unit polygons, deleting the original lines as necessary.
        If in constructing a second Bpoly you use a pre-existing Boly boundary, it must  be transect or be transected by the other Bpoly boundaries. Parallel boundaries are not  adequate to the Bpoly construction.


        Open the .dwg geological map you created earlier from the Fieldlog dataset,  and outline the area to be preserved with a rectangle drawn with the DRAW RECTANGLE command.
         Enter the ERASE command prompt and type ALL at the select objects  prompt. Next, do not hit the  ENTER key but type R (for remove entities not to be erased), and then CROSSING.
        Drag the `crossing rectangle' to just fit your bounding rectangle, and press `ENTER'.  All entities outside the rectangle that are not connected to entities inside the rectangle  will be erased.
        Enter the TRIM command and in response to `Select cutting edges(s)... Select  objects'   , click on the rectangle boundary as the cutting edge and press ENTER. At  the `select objects to trim' prompt, type F for FENCE and draw a line around the  outside edge of the rectangle as close to the rectangle as you can without crossing the  rectangle boundary, and press ENTER. (The Fence does not have to be closed - it could  be just a single straight line - but should cross all the lines to be erased. If all the lines  do not get erased, type F again and repeat the operation. You may have to carry out this  operation several times.)
       If you have removed parts of polygons with the trim operation it will be  necessary to use BPOLY to re-define the polygons affected.
        [Note: The same result can be obtained using the module 'section.lsp' written  by Bob Jones. Enter `appload section.lsp' at the command line, followed by the  command SCB (erase and trim outside a specified border and leave the border); SC  (erase and trim but do not draw a border; or SCD (erase all entities inside the border)].


        This is not a necessary operation if you intend to use FILTER to ouput DXF  files. However, to get rid of unwanted layers it is necessary to erase all entities from the  selected layers by turning them ON and FREEZING all others, and then ERASING  ALL. Save the drawing file, exit Autocad, reload Autocad and the file, and then run  PURGE. Autocad will then request permission to eliminate each individual layer that it  finds without data.
        There is one other way but you must be very careful as this method will purge  not only ALL unreferenced layers but ALL unreferenced blocks, text styles, dim styles,  linetypes, and all other table type data. It will also reset some system variables and I'm  not sure what else.

        If you only want the graphical information out of the drawing then:
        open the drawing to be purged. WBLOCK the drawing onto itself (give the  WBLOCK the same name as the original drawing file), select all entities in the drawing,  type '(getvar "insbase")' at the insertion base point prompt, and then quit the drawing  without saving the changes.  This will successfully purge the drawing.

        Double click button 1, a window will open in the top left hand corner.
        Click 0 to size the window, and 0 again to record the selection.
        Click 1 to initiate the zoom.


Figure 1. Plate Tectonics.


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