Evaluating spatial relationships - GIS Tutorial

- If you want to know how many six-inch water lines are in your city, query a database. If you want to know where the six-inch waterlines are that are in danger of corroding from running through highly acidic soils, ask a GIS. Because location is a part of every GIS dataset, your GIS knows what things are near each other, what things intersect, and even which polygons contain which points on the ground. So in the case of water lines, what you'd really be doing is giving your GIS a water line and a soils layer and asking the GIS to find all the water lines that intersect the acidic soil polygons. In the US, you can get that soil data for free from the US Geological Survey or from your state's department of natural resources. Intersects are one of the many spatial relationships that GIS is able to understand such as within a distance of, contains, are contained by, and shares a boundary with. Let me show you a map of Louisville, Kentucky. It's got fire district polygons on it. And when you zoom in, you can see there are address points. Suppose the fire chief wants to know how many homes and businesses the department is responsible for protecting. That's a simple are contained by question for GIS. Select all the address points that are contained by my fire district. Now suppose there's going to be construction on one of the streets in Louisville. I'll say it's Preston Street. The fire department might want to know how many homes and businesses are near Preston Street, which they might have trouble getting to because of the construction. So you could run a query to select all the address points that are within a distance of 500 feet from Preston Street. But Preston Street is a long street and runs outside of your district. So you could then run a spatial query to say out of all of these Preston Street addresses, just pick the ones that are in my fire district. And then you could open the table and see a list of all the addresses that would be affected by the construction in your district. And you could further query to see if any of those addresses are hospitals or schools. Then maybe create a mailer to go out to let everyone know that you've got a plan to take care of them even when the construction is going on. These address points are created and maintained by emergency response organizations around the world. There are also data vendors who sell address points with demographic data behind them. So you can do queries like which addresses are within one kilometer of my grocery store because I want to send them coupons or which addresses within one kilometer of my grocery store likely have school-aged children living in them and have a median household income greater than some amount. Spatial analysis gives you a new way to explore your data to pull out valuable information.