Map Analysis (Autocad Map) - Buffers and Overlay Operations

 Aim: to determine areas suitable for a groundwater well site in a 16 square kilometre region where the river system is polluted, and in which there are also a number of point pollution sites. Further, the well must be located in an area where the surficial sand and gravel layer is more than 30 meters thick. It is assumed that the groundwater contamination is present within 150 metres of the river banks and 500 meters of the point pollution sources.

 The strategy adopted is as follows:
1) create topology layers for each of the drawing objects;
2) generate buffer topology maps from the river and polluted point source topology;
3) attach an object data table containing a sand thickness field (thickness) to the 30 metre isopach
     line and create a sandisotop polygon topology;
4) combine the river and point pollution buffer zones (union overlay = Boolean 'OR’ condition);
5) subtract (erase) the resulting topology from the underlying 'base' boundary map;
6) add the further restriction (intersect overlay = Boolean 'AND' condition) that wells only be
    drilled in areas where the sand layer is more than 30 m. thick.

Initial Drawing Files:
Boundary.dwg -> create a polygon topology; this represents the "base" map from which unsuitable areas will be "subtracted".
Ppoints.dwg -> create a point topology; polluting point sources, e.g. septic tanks.
Rivers.dwg -> create a network topology; river source of pollution.
Sandiso.dwg -> create a polygon topology; areas where sand beds are greater than 30 meters thick.
Roads.dwg -> no topology; not used in the topological analysis.


Make a folder called Pollution in d:\ACADMAP2\mapanal.
Load Autocad Map and start a new drawing.
In Format -> Point style select a point style as the default point size.
Attach the drawing to the tablet and calibrate the tablet to the rectangle on the drawing, bottom left corner 0,0 to top right corner 4000,4000.


 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. Click on a second known point on a diagonal line from the first, and enter the coordinate value of the point. Press the ‘ENTER’ key in response to the request to enter a third point. 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’ or use the mouse if the tablet uses the WinTab driver. (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.

 IMPORTANT: each time you make a new drawing it will necessary to RECALIBRATE the tablet!!

Create a layer called boundary, and draw a 4000 x 4000 rectangle.
Save the drawing as boundary.dwg in d:\acadmap2\mapanal\pollution.

 Make a polygon topology called boundtop on a layer called boundtop.
 Map -> Topology -> Create -> give name 'boundtop' in 'Define Topology' and select 'Polygon' as 'Type' -> click 'Link Objects' button -> click 'Select Automatically' and then OK -> click 'Centroid Objects' -> click 'Select Automatically' -> in the 'Object Creation' box give name of the layer as 'boundtop' -> click OK -> click Proceed. Examine the topology.
 Save the drawing as boundtop.dwg in d:\acadmap2\mapanal\pollution.

RECALIBRATE the tablet and start a new drawing. Select Tools -> Object Snap Settings and click the 'Nearest' button. Intersections of river branches when drawn will now terminate exactly at the intersection of the rivers. Draw the river system on a layer called rivers, and save the drawing as rivers.dwg.
Clean up the drawing with Map -> Map Tools -> Drawing Cleanup.

 Make a network topology called rivtop and save as rivtop.dwg.
 Map -> Topology -> Create -> Enter the name 'rivtop' for the topology in 'Define Topology' and select 'Network' as 'Type' -> click the 'Link Objects' button in 'Link Objects' -> click 'Select Automatically' in 'Link Objects' and then OK -> click the 'Node Objects' button -> click 'Select Automatically' in 'Object Selection' -> click 'Create Node Objects' in 'Objects Create' and give the name rivnodetop as the layer name -> click OK -> click  the 'Proceed' button.
 The river topological data is attached directly to the lines (links) delineating the rivers on the rivers layer; there is no separate rivtop layer. However, if node objects marking the beginning and end of the river links are created, they can be saved to a new topological layer
 NOTE: it is not necessary to create a node topology in order to make a river buffer topology.

 Make a 100 meter buffer topology called rivbufftop on a layer called rivbufftop.
 Map -> Topology -> Buffer -> give rivtop as the input topology name and enter the value '100' as the 'Offset' -> give rivbufftop as the name of both the Result Topology and of the Topology layer -> click the 'Node Blocks' button and then the 'Proceed' button -> Regen the drawing.
 Save the file as rivbufftop.dwg.

RECALIBRATE the tablet and start a new drawing. Draw a system of point pollution sources on a layer called ppoints, and save as ppoints.dwg. Following the above procedure to make a topology called ppointstop, and a 500 meter buffer topology called ppointsbufftop on a layer of the same name.

RECALIBRATE the tablet and start a new drawing. Make a layer called sandiso, draw polygons delineating areas where the sand thickness is greater than 30 meters. Attach an object data table called sandiso containing a sand thickness field (thickness) to the 30 metre isopach lines (Map -> Object data -> Define Object data -> 'New Table'). In the present case this last step is not strictly necessary, but do it anyway for the practice; see *** below. Save the file as sandiso.dwg.
Create a sandisotop polygon topology on a sandisotop layer, and save as sandisotop.dwg.
 [***If you had initially drawn several isopach lines representing different thicknesses on the sandiso drawing, you would have had to attach the sandiso.dwg file and carry out a query with a condition set that the thickness of the layer be greater than 30m (Query -> Define Query -> Query Type -> Data -> Data condition 'Thickness = 30'). This would retrieve only the 30 isopach lines, each of which have been converted to a polygon by the addition of the boundary rectangle and the use of Drawing Cleanup.]

RECALIBRATE the tablet and start a new drawing. Make a layer called roads, draw a set of roads and save as roads.dwg.

 Start a new drawing file pollution.dwg, make an alias for d:\acadmap2\mapanal\pollution and attach the boundtop.dwg, the rivbufftop.dwg, the ppointsbufftop.dwg, the sandisotop.dwg, and the roads.dwg files.

Query -> Define query ->  Query Type -> Location -> Boundary Type = 'all' OK -> click 'Draw' in 'Query Mode' -> click  'Execute Query' button,  to import the contents of all the attached files.

 Freeze all the layers except the rivbufftop and ppointsbufftop layers.

From the rivbufftop layer and the ppointsbufftop make a Union (boolean 'OR') topology called pntsrivbufftop on a layer pntsrivbufftop. The resulting image will show those areas within the original defining rectangle that are not polluted according to the criteria used to define a polluted area.
 Map -> topology -> overlay -> load the files into the requisite boxes. If the 'Source' and/or 'Overlay' dialog boxes in the 'Input Topology' section are empty or do not contain the required topology names, click the Source dialog box followed by the LOAD button, and get the rivbufftop file from the scroll down list in the 'Topology Name' section . Repeat to enter the ppointsbufftop name in the Overlay dialog box. Enter the name of the 'Result Topology' as pntsrivbufftop and create on layer pntsrivbufftop. Click the Proceed button NOT the OK button..

 Unfreeze the boundtop and pntsrivbufftop layers and freeze all the other layers.

Use the Erase overlay operation to create a nopoluttop topology on a nopoluttop layer with the boundtop layer as the 'Source' and the pntsrivbufftop layer as the 'Overlay'. Make the nopoluttop layer red so that you can compare the pntsrivbufftop and nopoluttop layers.

 Unfreeze the nopolluttop and sandisottop layers.

Use the Intersect (Boolean 'AND') overlay operation to create a bestbet topology on a bestbet layer from the nopolluttop and sandisotop layers.
Note that two of the point pollution areas that formed islands within the eastern area of non-polution in the centre of the area remain as part of the bestbet topology. To remove these areas select Map -> Topology -> Edit -> select the topology -> click the Topology and Delete buttons -> click OK -> sequentially select the polygon topology nodes on the map to be removed. Press ENTER and then click CLOSE to terminate the operation.

 Freeze all layers except the road layer and carry out a drawing cleanup. Create a Network topology for the roads, followed by a road buffer with an offset of 300 meters Then carry out an intersect operation on the road buffer and 'bestbet' topologies in order to display all suitable areas for a groundwater well site within 300 meters of a road.