Week 9: Vector Analysis, Part II

This week's lab was all about creating and manipulating buffers around lines (roads and rivers) and polygons (lakes). Given a set of shapefiles for roads, rivers & lakes and for conservation areas, we had first to create buffers of varying distances around each of the three features (roads, rivers, lakes). Second, we had to combine these buffers through a "union" function, and finally, we were to create a new shapefile that included all the areas that were a) within the different specified distances of each of those three features but b) not within six conservation areas. a) was represented by performing an "intersect" function on the combined buffers, and b) was achieved by performing an "erase" function on the result of a). Along the way there were a few other manipulations of buffers.

This was a pretty straightforward lab for the most part and it was interesting to see that we could create buffers of different distances for different attributes within the same feature. My greatest difficulty was working with ArcGIS to try to calculate the areas of the final polygons because I couldn't initially get the Area field to be wide enough to include all the numerals. Eventually I tricked it (or so it seems to me), by giving the field a longer name, into giving me the answer I needed.

Answers to the lab questions:

1) Using "Intersect" gave me the same result on the map as using the records within the union of Roads and Water that were within both the roads buffer and the water buffer. The table for this layer contained four records, like the buffers_union_export file (before that file was converted to a singlepart layer, which had 82 records).

2) To exclude conservation areas from the project, I used "Erase", because it cuts out the conservation areas that are within (that intersect with) the buffer area.

3) The possible_sites layer, the final result, has four records, but two of them are the same on the map (in the table they are identical except that one has a buffer distance of 150 and the other of 500, and they have different FIDs); this part makes me wonder whether I have done something wrong as I don't understand exactly why I have (almost) duplicate records. The area of the largest feature is 60,783,617 square meters; the area of the smallest (duplicate) feature is 585,748.