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U of M FR 3131 - Projecting Geographic Data

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Lab 2: Projecting Geographic DataObserving How Distance Changes with the Map ProjectionLab 2: Projections 1 Lab 2: Projecting Geographic Data What you’ll Learn: In this Lab we will describe the basic methods for using map projections and datum transformations in ArcMap. The readings in Chapter 3, Map Projections and Coordinate Systems, of the textbook GIS Fundamentals, provide the necessary background. Data are located in the class web site & the \L2 subdirectory on the S:\ drive for the class in the Skok 35 & Green 210a teaching labs. Data includes Minnesota county boundary shapefiles and a lakes dataset, both in various projections. What You’ll Produce: A map of Minnesota in three different statewide projections, a map of reprojected county boundary and lakes data in central Minnesota, and a worksheet recording areas and coordinates for various projections. Background: The Earth's surface is curved in every direction. We introduce unavoidable distortion when we flatten this curved surface onto a map, typically changing areas, lengths, and the shapes of features. Different map projections introduce different types of distortion, and organizations choose the projection which limits distortions to levels they can accept. Map projections also represent the same points with different X and Y (or E and N) coordinate values. We cannot mix map projections in an analysis, so we often have to re-project some of our data layers. Each lab assumes you have a copy of the needed data files on your personal “jump” drive. Before each lab copy the needed files from the class web site or lab shared drive, S:\. Links for the class web sites are as follows: http://paulbolstad.cfans.umn.edu/Courses/FR3131/FR3131.html or http://paulbolstad.cfans.umn.edu/Courses/FR3131/FR5131.html Observing How Distance Changes with the Map Projection Start ArcMap, and add two data frames. Name one Albers, and the other Mercator (see last week’s lab or the video Data Frames for instructions) Activate the Alber’s Layer Add the layers twocity_Albers.shp, and USA_48_Albers.shp. Left click on the Measure Tool to enable it, and set the Distance Units to Miles (see Lab 1, or video Measure Tool.mov) Left-click once on Los Angeles, then move the mouse to New York and double left-click on New York. The distance between the two cites is displayed, either in a drop-down window, or at theLab 2: Projections 2 ArcToolBox Button Cursor coordinates bottom left of the ArcMap window (it depends on the version and setup). Your measured distance should be approximately 2,440 miles. Activate the Mercator data frame. Add the layers twocity_Mercator.shp, USA_48_Mercator.shp Re-measure the distance from LA to NY. The new measurement should be approximately 3,127 miles. The on the “ground distance” between LA and NY is actually 2,444 miles. The difference in measurements between the “Albers” and “Mercator” is due to unavoidable distortion caused when we stretch measurements from the curved Earth surface to a flat map surface. Projecting Shapefiles You often need to project data from one coordinate system to a different coordinate system. We will perform three different projections, and produce one map illustrating the differences between the separate projections. We will also look at the resulting differences in the measured area for one feature (in our case a county) in each projection. Start ArcMap, create a new empty map, and rename your data frame from “Layers” to “Minnesota Counties.” Place the L2\minn_county.shp file in your data frame. You should see a county map of Minnesota displayed in your screen, similar to the figure at above. Note the location of the ArcToolbox button and the cursor coordinates. Remember, because the toolbars are moveable, they may be in different locations than those shown. Move your cursor around the screen and notice the coordinate values to the lower right. Note how these change along with the cursor position as the program displays the map projected coordinate values corresponding to the cursor position. These data in the minn_county shapefile are in UTM NAD83 projection. Each coordinate value isLab 2: Projections 3 measured in meters, so a value X = 512,349 indicates an X value of 512,349 meters to the east of the origin. Note that most data layers have information stored that identifies the appropriate coordinate system. For example, the data set above is stored in the UTM, NAD83 Zone 15 coordinates. I might have another data set of the Minnesota county boundaries which is stored in geographic coordinates (latitude/longitude), or in state plane Minnesota South Zone coordinates, or another in an Albers coordinate system. I may convert one data set to another through a projection. If you reproject these data layers correctly, they will align properly. Note that you have the option of creating a permanent reprojection or a temporary reprojection with ArcGIS. This can be quite confusing at first, so read this section carefully, and make sure you understand it before you go on – you will likely save yourself much confusion and grief. Data Frame Coordinate Systems ArcGIS allows a data frame to have a coordinate system. Any data that is suitably documented and is subsequently placed in a data frame is converted to that data frame’s coordinate system “on the fly”. This means the coordinate projection is applied to the data read from the disk, but before it is displayed. This projection is “temporary” in that it doesn’t affect the data stored on the disk – it only reprojects the data temporarily, for display. This allows us to display many data sets in a data frame even if the data sets are stored in different map projections, without having to go to the trouble of manually reprojecting each data set and saving a new version of the data set. The “catch” comes in that when you first create a data frame, the coordinate system for the data frame is undefined. If you do not explicitly set the coordinate system for the data frame, it then takes the coordinate system of the first data set displayed in the frame. All subsequent data are then displayed in this “first” coordinate system, unless you manually override this data frame coordinate system. A few examples will clarify this. You might also want to look at the video, Intro to Projections. Create a New Map; (FileNew, “Blank Map”, no need to save


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