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Unformatted text preview: Geography 387 – Fall 2011 Lab 2 Map Projections 1 Lab 2: Map Projections 1. Purpose In this lab, we will assess map projection differences for the conterminous US. We will create a map layout with multiple data frames in it (four maps in one page), and we will create a new feature class in different map projections. Aside from subtle visual alignment differences, each projection's areas will be different. We will compare the spatial differences by "calculating" the area of the US and a special test shape. 2. Brief overview of coordinate systems and projections in ArcGIS Geospatial data are encoded with certain units. The spatial components of each feature class (lines, points) are projected using a mathematically defined coordinate system transformation. The earth is round and maps are flat. Map projections flatten the round earth. ArcGIS recognizes a bewildering number of projections and coordinate systems, but in practice only a limited subset of these are used. It is often the case that geographic data are converted from one projection to another – or, to put it differently – to convert data that is ‘flattened’ one way into data that is ‘flattened’ another way. ArcGIS has attempted to simplify working with data in different projections by “projecting on the fly”. Essentially ArcGIS will try to recognize what projection your data have and then convert the data to the projection you are working in. The “on the fly” part means that ArcGIS does not make the conversion permanent. Usually this works, sometimes it doesn't. Often, it misleads you into thinking all your data are in the same projection when they are not. When you add data to ArcMap it is ‘contained’ in a Data Frame. Recall from the first lab that there are two view options in ArcMap: one is Data View and the other is Layout View. When you are in Layout View you can see the Data Frame that contains your data. Think of the Data Frame like a window on your data. You can resize the frame and move it around in the layout. You can also add other Data Frames to the same map layout. Each Data Frame has its projection it uses to draw the feature layers in it. You can change the projection of the Data Frame but this does not change the actual projected units coded in the feature class – it only changes the projection that you are viewing the data with - in that Data Frame. This is the “on the fly” part. Some of the features used in this lab have no projected coordinates, but rather have feature coordinates stored in latitude and longitude spherical coordinates (degrees, minutes, seconds, or decimal degrees). This is called the Geographic Coordinate System (GCS). If, however, for calculating areas or precise distances, this is not an appropriate coordinate system. You need a flat planar system, i.e. you need a Projected Coordinate System to do this. Square degrees are meaningless, and for most people a distance specified in degrees is also meaningless. All projection operations are based on spherical coordinates, and so this lab begins with feature classes in GCS, and then moves to transforming these data into projected coordinate systems. classes in GCS, and then moves to transforming these data into projected coordinate systems....
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- Fall '08