Sunday, May 15, 2016

Module 02 - Queries and Clips

In Module 02 we learned to perform SQL queries and to use the Clip tool of ArcMap. In this module we used these techniques to compare the areas and landmarks of Chicago before, and 20 years after the Great Chicago Fire.

The SQL query is a very useful tool for selecting a range of data from a larger database. The query is done by inserting an equation referencing file's attribute table into the tool. Once the query is run, the newly selected data can be exported into a separate shapefile. This provides a quick and relatively easy way to separate a large shapefile into a number of smaller ones that may offer more specific information. In the case of the Chicago Fire map, I have taken a shapefile with landmarks in Chicago, and created two separate file, one with all the landmarks build before the fire, and one with landmarks build between 1871 and 1890.

The Clip tool simply takes a shapefile, and removes all data from it that falls outside of another, related shapefile. In the case of the Chicago Fire map, once I had the pre 1871 landmark file, and 1871-1890 landmark file, I clipped to their respective city shapefiles. The pre 1871 landmarks file was clipped to the Chicago 1869 map, thus removing all landmarks outside the city limits. The 1871-1890 landmark file was clipped to the Chicago 1890 map, again removing all landmarks that fell outside the city limits at the time.

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