Mapping and interpolating 2D data in GIS Dr. Mark J. Brush MSCI 503

Transcription

1 Mapping and interpolating 2D data in GIS Dr. Mark J. Brush MSCI 503 I. Making a station map 1. Open ArcMap you ll see the window below (left figure). Click the Catalogue button and select the Connect to Folder button in the toolbar (right figure). Navigate to the folder where you have stored the GIS files, select it, and hit OK. Close the Catalogue window when done. 2. Geographic data (coverages, shapefiles, data layers) can be added with the Add Data button. I have placed different versions of the Chesapeake Bay coastline on Blackboard. Add the file shore_dd. 3. Symbology can be changed by double-clicking on the legend or rightclicking and selecting Properties on the heading. 4. An alternative background map can be made using the baybox_dd shapefile. Add that to the map. Under Properties find the Symbology tab and select Categories and choose Unique Values. In the drop-down box for Value Field select the last option and click the Add All Values button at the bottom. Click Apply and you ll see one color for land and another for water. You can now double click on each color and change them; e.g. make land grey and water hollow.

2 5. Now add stations click Add Data and select the file station data.xls and select the worksheet stations$. Once it appears in ArcMap, right click on the file and select Display x-y data Make sure the x and y fields say long and lat and hit OK. These are the long-term Chesapeake Bay Program stations in the York River. Zoom into the York. 6. Select the Identify button and click on a station. The Identify window will appear. If the stations file is selected in the drop-down box at the top of this window, you will see the data associated with that station, including station name, approximate depth, and some rough chlorophyll-a data I added. 7. Again, station symbology can be changed. Right click stations$ Events, select Properties, and choose Quantities on the Symbology tab. You can make the symbol color (Graduated colors) or size (Graduated symbols) a function of chl concentration by choosing chl in the drop-down box under Fields Value. (Click Apply to update the map.) The color ramp can be reversed by clicking on Symbol in the table header and selecting Flip Symbols. 8. You can add labels by right clicking on the data layer and selecting Label Features. To change which column is used as the label or modify fonts, select the Labels tab in Properties. 9. If any of your layers fail to show up on the map, check their order under the Table Of Contents section (see figure at right). Layers are drawn in the order you see them in this section. To change the order, select the first button (red box) and simply drag the layers around.

3 II. Interpolating station data 1. Click the Arc Toolbox button (see Fig. 1). In the window that opens, select Spatial Analyst Tools Interpolation IDW (see right). Double click IDW (this is also where other interpolation options are like splining and kriging). 2. Fill out the window that opens with your chosen settings (see help files and the reading I put on Blackboard about interpolation settings). In the example below, I am using my values for chl in the stations file, the default grid size (hard to interpret since we re in decimal degrees, but see below), and having ArcMap base its interpolation in each grid cell on the values at the nearest 5 stations (i.e., all the data). Click OK. 3. As above, you can change the color scheme, intervals, etc of the resulting interpolation of chl-a on the symbology tab (right click on the layer Properties). Try using the stretched color bars. If you move the order of the layers under Table of Contents so that this interpolated layer plots below baybox_dd with filled land and hollow water, you will get a nice map. 4. You can create contours of the interpolated chl-a layer by going to Arc Toolbox Spatial Analyst Tools Surface Contour.

4 III. A note on coordinate systems If you look in the bottom right corner of the ArcMap window, you will see that the coordinates for the map we have been making are decimal degrees (which is why my shapefiles end in _dd ). GPS coordinates you collect in the field will be in degrees (D), minutes (M), and seconds (S). You will first need to convert these units to decimal degrees as follows: Decimal Degrees D M S Important: In the Western Hemisphere, you need to make longitude negative, e.g for it to plot correctly in ArcMap. Decimal degrees work well for most applications, but there are many other coordinate systems. The most common one is the UTM (Universal Transverse Mercator) system, which expresses location in meters in 60 equally-spaced zones around the globe (see figures below). Latitude (utm-x) is expressed as meters from the equator (+ in No. Hemisphere; - in So. Hemisphere). Longitude (utm-y) is expressed as meters (always +) by assigning a value of 500,000 m to the centerline of each zone. We live in UTM zone 18N. Since the units of UTM are meters, when you interpolate data, the grid cell dimensions are given in meters, which makes it easier to set and interpret the values. Source: ems-i.com Source: Idaho State University

5 IV. Converting from decimal degrees to UTM and visa versa 1. You can convert between coordinate systems (called map projections) with Arc Toolbox Data Management Tools Projections and Transformations Feature Project. For example, if you want to work in UTM and you create an Excel file of your stations in decimal degrees, you can convert it to UTM, use the shapefiles I have provided in UTM (shore_utm, baybox_utm), and interpolate in UTM. 2. See the example below for converting the station file we have been using into UTM. I added the stations file under Input Dataset. ArcMap already knows its coordinate system. In some cases you ll have to exit out and first define the projection using Define Projection under Projections and Transformations. I shortened the name of the output dataset (otherwise it was too long and the projection failed). 3. For Output Coordinate System, click the button to the right, click Select, and you ll see the possible coordinate systems you can use. For UTM, I chose Projected Coordinate Systems UTM NAD 1983 NAD 1983 UTM Zone 18N.prj. Click OK twice and a projected stations file will be added. You ll need to use File New, create a new map, and add only the files in UTM. You should now see values in meters in the lower right corner of the window. 4. FYI, when you are in Projected Coordinate Systems you can take a look at the many other options you have. Decimal degrees are under Geographic Coordinate Systems.

6 The following website also provides an Excel sheet for converting between UTM and decimal degrees: V. Plotting Dataflow data vs. distance 1. If you have Dataflow output you will have an Excel file with coordinates in decimal degrees. Convert these into UTM in ArcMap, or use the follow approximate conversions for the York River: utmx = (long*88575) utmy = (lat*110951) You can now compute the approximate distance between successive samples as (call the first point at 0,0): meters = previous position + (SQRT((x 2 -x 1 ) 2 +(y 2 -y 1 ) 2 )) VI. Finalizing maps for figures: 1. Under the View menu, select Layout View. 2. Legends, scale bars, north arrows, etc. can be added under the Insert menu. 3. You can add graticules and grids (the border of a map with lat/long labels) by selecting View Data Frame Properties Grids tab New Grid. 4. Use File Export Map to save as an image for use in a figure. 5. Locus Maps: It is often nice to add a locus map, which is an inset showing the wider region with your study site highlighted in a box (see my example on the next page). Go back to View Data View and select Insert Data Frame. Highlight this new data frame under Table of Contents and add a shapefile of the broader region (e.g. shore_dd). This will now plot on the Layout view (go back to View Layout View) and you can place it where you want it. VII. Selected GIS data sources: There are several sources for GIS shapefiles and coverages on the internet, where you can get files of other coastlines, state boundaries, etc. Some of the most common sites are:

7 General (coastlines etc) Land Use / Land Cover Elevation Bathymetry Hydrography (rivers, lakes, wetlands) National Wetlands Inventory US Census Data Chesapeake Bay Program (local bay files) ftp://ftp.chesapeakebay.net/pub/geographic/