Click her to go to 'Figures/Overheads' section.
Click here to return to course outline.
'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.
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.
SELECTING A RECTANGULAR SECTION OF A MAP TO OUTPUT
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)].
PURGING UNWANTED LAYERS
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.
THE PANACEA ZOOM FUNCTION
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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Click here to return to course outline.