Lab 4. Extracting line segments from Polygons

 

You may need to take lines that you created as part of your polygon layer that defines units and use these as part of your fault layer.

 

For example, you have two polygons picture above.  You want to separate the individual lines so that you can use the boundary as a fault line and as the boundary line, as seen below.

 

This view has the fault line that forms the boundary as a different line style from the rest of the polygon.  In your maps, you will use the fault line style.

 

How do you do this?

 

            The process is several steps and uses both ArcGIS and Arcinfo.  Arcinfo is the command line interface that we discussed earlier in the semester.  It has far richer set of commands than ArcGIS but is limited to command lines, which means you need to know commands to do this.

 

            First you need to make a layer for working with in Arcinfo.  You will use ArcCatalog, which you used to make the shapefiles you used for digitizing the units.  Instead of making a shapefile, you want to make a coverage.  Name the coverage incoming.  A coverage is similar to a shapefile in that it stores spatial information, but is more complex.  It is the native format for Arcinfo.  ArcGIS can display and work with shapefiles and coverages.  The coverage should be a line coverage, so that you can select lines from your polygon layer and then send them to this new coverage.  This is the first window:

 

 

            You don’t need to use a template coverage.  Click the Next button, which brings up this window:

 

 

            Click on Next, which brings up the projection window:

 

 

            You should define a projection interactively.  Click on Next.

 

 

            Select the UTM projection, then click Next.

 

 

            Change the zone to 11 and click on Next.

 

            Change the datum to WGS84 but don’t select a spheroid then click Next.

 

 

            Click on Finish to complete the projection.

 

 

            Click on Next.

 

 

            Change the base feature class to Line then click on Finish.  You are ready to start Arcinfo.

 

Once you have made your coverage, start Workstation Arcinfo.  You can find it under the ArcGIS tab in the Start menu.  Arcinfo starts a new window, which looks like this:

 

 

It can be a little intimidating since it doesn’t have a GUI but you will have all of your commands here so be patient.

Once you have the window open, you need to change to the folder you are working with.  The command you use is “w”, which changes workspaces, or folders.  You need to give the complete path to where you stored your new coverage.  Your command might look like this: 

w   C:\tempshare (this should be were you saved the new coverage and where your polygon shapefile is)

Once you are there, you need to import your polygon coverage in Arcinfo from the shapefile format.  This the polygon shapefile, not the new line coverage you just made in ArcCatalog.  Do this with the shapearc command, which should look like this:

shapearc polygons.shp polylines

This creates a new coverage, called polylines.  You then need to clean this, which takes all of the lines that define the polygon and separates them.  The command looks like this (given the name of the new coverage I created above):

clean   polylines   #   0.001   0.001   poly

Now you are ready to use the Arcinfo editing tool, ArcEdit.  Start Arcedit with the arcedit or ae command:

ae

Now open a display window so you can do visual selections.  Use the display command:

display   9999

You will get a simple display window that looks like this:

 

Now you need to establish some parameters, such as the layer you are editing and the type of data you want to edit.  You do this with the editcoverage command (ec) and the editfeature command (ef).  They should look like this:

ec    polylines     (which is your polygon coverage, which you converted using the shapearc command)

ef   arcs

You also need to draw the data on the screen, so that you can see it to select it.  The drawenvironment command (drawe) tells Arcinfo which parts of the coverage to draw.  The draw command tells Arcinfo to actually draw it:

drawe arcs

draw

Now you can select lines, use the select command:

select

Once you type the select command, use your cursor to click on a line.  The selected line will be redrawn in yellow.  Once it is selected, you can save it in your incoming coverage using the following command:

put incoming

This appends your selected line to the new coverage you have built.  You can do this repeatedly.  You will be prompted on whether on or not to append.  Your answer should be yes!

Once you are through working, you need to convert this file to a shapefile.  Do this with the arcshape command:

 

arcshape incoming arcs newshape.shp

 

Now you are ready to add this new shapefile (newshape.shp) to your map and change the symbolization to reflect the type of lines.  You can also edit the lines and add columns that you can use to designate fault types, etc.

In order to do this, add a new field to the faults.shp, named type. Label each one of these lines as thrust. We'll do the labeling in class.