Harry Williams, Cartography 1

INTRODUCTION TO GIS

A Geographic Information System is a combination of software and hardware that can store, manipulate, analyze and display geographic information.

What is geographic information? Any information that has spatial coordinates (can be mapped).

A common approach is to represent different kinds of spatial information as separate LAYERS within the GIS.

Harry Williams, Cartography 2

How do you do GIS?

1. Learn to use a software package (usually takes 15 weeks out of a 16-week semester).

2. Get your data into the GIS.

3. Conduct spatial analysis to answer some question.

Because we have limited time, we will bypass step one and focus on

an aspect of step 2 and step 3.

Harry Williams, Cartography 3

Getting your data into the GIS – Geocoding.

Many databases contain street addresses – this qualifies as geographic information (can be mapped) and can be represented as points on a GIS map.

Portion of UNT database containing student addresses.

Harry Williams, Cartography 4

Geocoding is made possible by street maps with built-in address ranges for each street segment (segments correspond to blocks). When an address from a database is matched to the street map, spatial coordinates are assigned to the address so that it can be mapped as a point. Geocoding the address "2,060 Georgetown" would result in a point about 6/10's of the way along the block on the right-hand side of the street (shown by the blue star).

Harry Williams, Cartography 5The street map

Harry Williams, Cartography 6The geocoded database.

Harry Williams, Cartography 7

The streets and the addresses as layers within a GIS.

Harry Williams, Cartography 8

Where do street maps with built-in address ranges come from? Many street maps used for geocoding are based on U.S. Census Bureau TIGER files (Topologically Integrated Geographic Encoding and Referencing). These files can be used directly, or software companies update these maps, correct any errors and make them available commercially.

Now the data is in the GIS what do we do with it? Ask a geographic question. For example, let’s assume the students are bus users and we want to see how many of them are more than 0.25 miles (straight-line distance) from the nearest bus stop on a proposed bus route for UNT students living in Denton.

Harry Williams, Cartography 9

Harry Williams, Cartography 10

A common way to do spatial analysis involving distance between map features is to use buffers. A buffer is a region constructed by the GIS to be a fixed distance from a selected feature. In this case, we will buffer the bus stops to a distance of 0.25 miles (note: in this case, there is no requirement to keep the buffers “separate”, so they will be merged together as one object within the GIS).

Harry Williams, Cartography 11

Harry Williams, Cartography 12

The GIS can now be used to select student addresses beyond the buffers (more than 0.25 miles from the nearest bus stop). This provides the answer to the original question and presumably would be used to improve the proposed route by adding more stops or changing the location of current stops.

Harry Williams, Cartography 13