Spatial Thinking in Planning Practice: An Introduction to GIS

17 duct a measurement of land parcel size, you need to use a projection that does not distort area space. Most GISs include functions to assist in the identi!cation of map projections, as well as to transform between projections in order to synchronize spatial data. Despite the capabilities of technology, an awareness of the potential and pitfalls that surround map projections is essential. ON(THE(FLY PROJECTION Creating map projections was extremely challenging, even just 30 years ago. And now we can project and un- project massive quantities of coordinates, transforming them backward and forward from Latitude and Longi- tude (assuming this or that earth model) to overlay precisely with data that are stored in some other coordinate space. It is truly amazing that humans have perfected a rich library of open-source so$ware that can Forward Project geographic coordinates (latitude and longitude, + earth model) to any projected system; and also back- ward project) from any well described projected coordinates back to geographic coordinates -- all in the wink of an eye. We can be thankful for that. But there are still some details that we have to understand. Automatic transformation of coordinate systems requires that datasets include machine-readable metadata. In about 2002, the makers of ArcMap added one more !le to the schema of a shape !le. "e .prj !le contains the description of the projection of a shape !le, and if it exists, it is always copied with the shape !le or dlements that are exported from it. "is is the machine-readable metadata that allows ArcMap to know how to handle the dataset if any transformation (reprojection) is required. "ere are plenty of datasets that do not include such machine readable metadata. "is includes data that are not created with ArcMap since 2002 and even some that are. So we should get used to understanding map projections and their properties. If you need to learn to set the coordinate system for a dataset, use ArcCatalog - as explained in the "e ArcMap Projections Tutorial. UNIVERSAL TRANSVERSE MERCATOR "e !rst coordinate system we want to introduce here is the Universal Transverse Mercator grid, commonly referred to as UTM and based on the Transverse Mercator projection. Universal Transverse Mercator (UTM) is a coordinate system that largely covers the globe. "e system reaches from 84 degrees north to 84 degrees south latitude, and it divides the Earth into 60 north-south oriented zones that are 6 degrees of longitude wide (Figure 2.6). Each individual zone uses a de!ned transverse Mercator projection (See Figure). "e UTM system is not a single map projection. "e system instead has 60 projections, and each uses a secant transverse Mercator projec- tion in each zone. "e contiguous U.S. consists of 10 zones (Figure 2.7). In the Northern hemisphere, the equator is the zero base- line for Northings (Southern hemisphere uses a 10,000 km false Northing). Each zone has an arbitrary central meridian of 500 km west of each zone’s central meridian (called a false Easting) to insure positive Easting values and a central bisecting meridian. In UTM, the CSUS Geography Department is located at 4,269,000 meters north; 637,200 meters east; zone 10, northern hemisphere. UTM zones are numbered consecutively beginning with Zone 1. Zone 1 covers 180 degrees west longitude to 174 degrees west longitude (6 degrees of longitude), and includes the westernmost point of Alaska. Maine falls within Zone 16 because it lies between 84 degrees west and 90 degrees west. In each zone, coordinates are measured as northings and eastings in meters. "e northing values are measured from zero at the equator in a northerly direction (in the southern hemisphere, the equator is assigned a false northing value of 10,000,000 meters). "e central meridian in each zone is assigned an east- ing value of 500,000 meters. In Zone 16, the central meridian is 87 degrees west. One meter east of that central meridian is 500,001 meters easting. Chapter 2: Coordinate Systems and Projecting GIS Data