Example: Bounded Areas

Given line objects, the Bounded Areas transform template creates areas in regions entirely enclosed by overlapping or otherwise touching lines.  This topic provides a quick look at how the Transform pane helps create new drawings.




We start with a drawing of Roman roads in France, as seen above as a layer in a map.   The drawing and the map are both in Lambert Conformal Conic projection, a meter-based projection.  We would like to create a drawing of area objects where each area neatly fills in a region enclosed by roads.   


The roads as seen above are formatted using a style that provides fatter, brown lines.  We have used the Layers pane to specify a beige background color for the map.  As is typical of drawings that represent roads, roads are often drawn using more than one line object, with the ends of the lines exactly coincident.




We choose the Transform pane, and we click on the Bounded Areas template.   If we like, we can click the Options button to rename the drawing and table that will be created to whatever names we like if we want something other than the defaults.   


We press the Add Component button.   A new table and drawing are created in the Project pane.




We drag and drop the resulting drawing into the map as a layer below the Roads layer.    We style the resulting drawing so that areas are light green in color and area borders are thin black lines.  We have also used the Layers pane to apply 50% layer opacity to the Roads layer.    The resulting display makes it easy to see the locations of the Roads layer lines and to compare where the Bounding Areas template created area objects.


We can see there are some regions that appear to be enclosed by the fatter, brown Roads lines which did not result in the creation of a bounded area.   That usually happens when there are small breaks in the road lines so that although the region appears to be enclosed by roads, it is not really enclosed.   We can Right-click and drag to zoom in to the end of one such line that appears to enclose a region.




The magenta arrow in the illustration above points out the break.  A line that at higher zoom appears to fully enclose a region in reality does not enclose the region.  Whoever created this drawing of roads made a mistake in this part of the drawing, and failed to draw the end of the line all the way to the obvious juncture nearby.


Of course, it could be that in reality the Romans built this particular road for many kilometers but failed to extend it all the way to the town that was likely located at the junction of the other roads seen nearby.   More likely is that whoever created this drawing simply made a mistake in drafting.


Such drafting errors are frequent in drawings of road networks.   People often create such drawings by either using tools that have poor snapping capability, or without using the snapping capability that is there.  Inexperienced or careless personnel may simply think that because a line seems to have been drawn to connect to the next line that it really does connect, when in reality it fails to touch at high zoom.




Finding such small breaks in a drawing where there should not be any breaks can be challenging.   In the illustration above we see another region that appears to be enclosed but which the Bounded Areas template does not reckon as enclosed.  One technique to find breaks is to Ctrl-click on lines to select them, and to then look at the ends of those lines at higher zoom to see if any breaks are apparent.




We have Ctrl-clicked on a line to select it, and now we can zoom box into one of the ends of the lines to see if it connects to the next line.




There appears to be a small gap between lines that seem like they should be connected.  We can zoom box even further to take a closer look.




Zoomed far in we see an obvious gap  If we measure the distance using the tracker tool, we can see the gap is about 60 meters.   This is nothing more than sloppy drafting, a typical error when GIS drawings are created.     Keep in mind that the gap above might not be the only gap that prevents the road lines from enclosing a region.   There could be other gaps as well.



Are gaps tedious? -  Yes, of course.   Finding and repairing such errors is one of the more tedious aspects of GIS work.   In theory, we could simply ignore such errors and force the creation of bounded areas by increasing the Tolerance parameter to some suitably larger value, such as 100 meters or 200 meters.   That is a brute force approach that might be OK in some circumstances.   But in the case of Roman roads that cover all of France, using a Tolerance parameter of 200 meters would also force together Roman roads, eliminating the area between them, in regions where Roman roads might run in parallel less than 200 meters apart, as they do in various formerly urban areas.   The basic problem is that it is very tedious to detect and to repair junk data which is full of errors on a scale that is similar to differences in legitimate data.  The best way to avoid such problems is to make sure that people who create such data are taught how to use tools, like snapping, that help avoid such mistakes.


Repairing gaps - There are two approaches to filling in a gap such as seen above.  One is to edit one of the existing lines to move the endpoint of that line so it is exactly coincident with the endpoint of the next line.  The other approach is to draw a new, short line to fill the gap, using snapping to ensure it starts exactly at the endpoint of one of the existing lines and then ends exactly at the endpoint of the next line.  Adding a new, gap-filling line has the advantage that it does not alter the original set of lines that were imported.  The new line can have an attribute field that indicates it is a gap-filling line that was added.    In cases where the original set of lines may have some scientific significance or where it is useful to preserve an audit trail of what was done previously, for example, if a contractor must be billed back for sloppy work, then it might be better to add a new line instead of adjusting existing ones.



Manifold 9 - Bounded Areas - The Bounded Areas transform template creates areas within regions that are enclosed by lines.   The template works with a single click, and automatically reckons any regions enclosed by either intersecting lines or by lines that are coincident end to end.  Easy!  Works in the free Viewer, too.


See Also



Transform Pane


Transform Templates - Drawings


SQL Functions