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.Geography 387 – Fall 2011 Lab 2 Map Projections 2 3. Data download and lab preparation Get the data Create a new folder for your Lab 2 work. Download and unzip lab2.zip into your work folder. Create a folder connection to the data. Open ArcMap and look at the feature classes for this lab You should see a file geodatabase named “us_parks_cities” with three feature classes in it. Recall the previous lab – from the Catalog window in ArcMap you can bring up the Properties of a feature class. Using the Properties window, find the XY Coordinate System of each of the three feature classes: 4. Setting the projection of a Data Frame Make sure you have a new empty map in ArcMap and then go to the Layout View. You should see a rectangle on the page with eight little blue squares at its corners and edges -- one on each corner and one in the center of each edge. The little blue squares, and the dashed blue line through them indicate that the Data Frame rectangle is ‘selected’. Question 1: What is the name of the XY Coordinate System of each of the three feature classes? a. cities b. USA_48 c. USA_parksGeography 387 – Fall 2011 Lab 2 Map Projections 3 Right-click the mouse inside this rectangle and select “Properties” from the popup menu. A window opens up whose title is “Data Frame properties”. It has several tabs. Select the tab for “Coordinate System”. This tab displays the coordinate system (geographic or projected) that the Data Frame is using. Since no data has been added to the Data Frame, it has no coordinate system. Under the heading for “Current coordinate system” it says “No projection”.Geography 387 – Fall 2011 Lab 2 Map Projections 4 Click on “Cancel” to close the Data Frame Properties window. Let’s look at the same Data Frame properties another way. Go to the TOC window in ArcMap and right-click on the “Layers” entry and select “Properties” in the pop-up menu. Notice that the window that pops up is titled “Data Frame Properties” – the same as the last step. Again, look at the tab for “Coordinate System” and see that the Data Frame has “No projection”. Click on the tab for “General” and notice that the Name of the Data Frame is “Layers” and that the Units for the Map are “Unknown Units”Geography 387 – Fall 2011 Lab 2 Map Projections 5 Click “Cancel” to close the Data Frame Properties window. This empty map document currently has one empty Data Frame, named “Layers”. You have just opened the Properties window and checked the projection and map units of that Data Frame in two different ways. Now, let’s add some data to the Data Frame. Drag the cities feature class from the catalog window into the map window. ArcMap draws the points that are in the feature class in the Data Frame. Notice that cities now appears in the TOC. Check the Data Frame Properties and find the projection that is now used in the Data Frame. And check the Units for the Map.Geography 387 – Fall 2011 Lab 2 Map Projections 6 We are going to repeat the process we did with the cities feature class only this time with the USA_48 feature class. Start a new map document (Hint: use the “New Map File” icon in the toolbar and don’t save the previous map you were working on). In