Vectors and Buffers

Vectors

This week's lab was all about understanding vectors and how to use them in spatial analysis.  I learned how to preform attribute and spatial queries to answer questions about data.  This process is useful for determining how close certain types of features are to other types (for example: how many schools in a county are within a mile of the nearest hospital.  Or what land parcels are more than a mile away from a railroad).  Multiple queries can be combined to make a more sophisticated question.  It is also possible to run queries for features that are completely within or only partially within other features, such as properties that are within a flood zone.

Additionally, it is possible look at the attribute tables from these selections to learn other things, such as the sum of the property values of the buildings within flood zones.  The spatial join tool takes this even further by taking attributes from one layer and appending them to the corresponding attribute from another layer.  For this lab we used the spatial join tool to find out which drainage basins have the most contamination risk features within them by joining a risk features layer to a drainage basins layer.

Buffers

In the second half of the lab we learned about buffers.  A buffer is an area of space around a point, line, or polygon.  Buffers can be variable, such as giving one type of body of water a larger buffer than another type of body of water.  Buffers can also be multilayered when it is important to see multiple distances around a particular feature.

For this lab we used buffers to determine ideal campsites in the De Soto National Forest near Hattiesburg, Mississippi.  We were provided with a layer for the roads, and another with lakes and rivers.  The best campsite has to be within 300 meters of a road and also within 150 meters of a lake or 500 meters of a river so that it is easy to get to but also near enough to water for activities such as fishing and swimming.

The first step was to create a variable distance buffer around the water features.  I added a new field to the water features table called "buffdist" with set the appropriate buffer distance for the body of water.  Then, I used the buffer tool to draw the buffer using "buffdist" to set the distance.  At this point I also created a buffer around the roads.

The next step was to create a union between these two layer to see the overlap between them.  This was done using the union tool to determine the intersect between the two of them.  To ensure that campers are discouraged from staying in areas with rare plants and animals with used a layer with the conserved areas and the overlay tool set to "erase" to eliminate the protected areas.

At this point, I now want to change are multipart polygons to single polygons so we can better assess each camping area.  The multipart to singlepart tool is designed for exactly this.  By adding a new field called area and using the calculate geometry tool I added the area of each feature in hectares.  I also split the polygon symbology into three equal intervals to help highlight the largest suitable campsite polygon.

Suitable Camping areas in the De Soto National Forest