Creating Grids, Contours and Hillshades in ArcView® Spatial Analyst®

Contours reveal the relief or topography of a region. Light Detection and Ranging (LIDAR) beach mapping data may be used to create detailed topographic beach maps. Before you can create topographic maps, you will need to acquire a LIDAR Beach Mapping data set. For this application be sure the data are downloaded in units of feet. These data sets can be extracted from two different sources. The BeachMapper software application provided on volume 2 of this CD-ROM set is one source for extracting LIDAR beach mapping data. For more information about the BeachMapper application, click here . The other source for acquiring LIDAR data is the LIDAR Data Retrieval Tool (LDART). This is a web-based data selection tool that allows the user to pick a state, area of interest, and projection before downloading the LIDAR data. For more information about LDART, click here. If you are unsure how to load a LIDAR beach mapping data set into an ArcView project file, please refer to the tutorial, Using LIDAR Data in an ArcView Project.

A note of caution: Depending on the computer system specifications (e.g., processor, disk space, amount of RAM), it may not be feasible to process large LIDAR data sets in ArcView Spatial Analyst. To ensure you are not going to overload the processing capability of your computer, check the file size of the LIDAR data text file you download. A data set no larger than 1 megabyte is recommended for this test of your system.

Due to the large amount of data being processed, the contour maps included on this CD-ROM set were created for entire islands using the ARC/INFO® GRID module. For more information on this process, please see the metadata records for the individual islands. Smaller areas may be contoured using ArcView Spatial Analyst.

Creating an Edit Mask

Once the data are loaded into an ArcView Project, an edit mask must be created. An edit mask is a type of grid that limits interpolation of the data to an area of interest or delineates a boundary where data actually exist. If you do not use an edit mask, ArcView will interpolate beyond the bounds of the data, creating an unrealistic surface. For example, if the highest elevation points inland are 15 feet in elevation, ArcView assumes that the upward trend will continue and could interpolate to 250 feet. While some coastal regions do reach 250 feet in elevation, we would not expect elevations much over 25 to 30 feet along the southeast Atlantic coast (unless tall buildings are included in the survey).

An edit mask can also be used to limit the interpolation area to the beach face. This is important because the 1997 LIDAR data set extends inland and includes buildings and vegetation. In most cases it is not relevant for coastal resource managers to contour buildings. Therefore those elevation points that are inland of the dune line can be ignored.

There are several ways to create a grid using Spatial Analyst. One method, which generates a solid grid and is therefore an effective edit mask requires several steps. You must first create a new polygon theme. This allows you to "draw" exact boundaries for the area to interpolate. The polygon must then be converted to a grid in order to serve as an edit mask. Although the process is a bit tedious, once you have created an edit mask, you can use the same mask for the 1996 and 1997 data sets.

Edit Mask Step 1: Creating a New Polygon Theme

From the menu bar, open the View pull-down menu and choose New Theme. A window will appear so that you can choose the type of theme. For Feature Type, choose Polygon. A window will appear so that you can name the polygon theme and give it the correct directory path. Here the new theme is named Mask.shp. The new theme will appear in the View window in "edit mode", with a dotted line surrounding the button. You will probably need to zoom to the extent of the data shapefile so that the entire data set is encompassed within the View window. You will be unable to zoom or pan once you begin drawing the polygon. Activate the new theme by clicking on the name once. From the tool bar, open the pull-down menu for the draw rectangle tool (last tool on right).

Choose the irregular polygon button. Place the cursor in the View window and a crosshair will appear. Using the crosshair, click near the outer edge of the area covered by the LIDAR data points. Trace the area covered by the data points, just slightly on the inside edge so that the interpretation will not go beyond the data boundaries. Each click will "pin" the perimeter of the polygon as you move along. The more clicks you make, the more precisely you can make the polygon follow the shape of the data points. Double click to complete the polygon. From the menu bar, choose the Theme pull-down menu. Choose Stop Editing. Click Yes to save and name the new polygon.

Edit Mask Step 2: Converting the Polygon To a Grid

Before the polygon can be used as an edit mask it must be converted to a grid. To begin, activate the polygon theme. Click the Open Theme Table button to open the table of the active theme. Click on the attribute table. Under the Table pull-down menu, choose Start Editing. Under Edit, choose Add Field. You can give the field a name in the window that appears. Here it has been named "Value." The other parameters may be left on their default settings. Click OK.

Using the arrow tool, select the record. It will be highlighted in yellow. Click on the Edit Button. This button allows you to change cell values. Click on the cell under the new field. It should have a value of 0 to begin. Change the value to 1. Use the Select None button to deselect the record. Under Table, choose Stop Editing; click Yes to save your edits. This step gives the area within the polygon a value so that it may be converted to a grid. Close the table and activate the Mask theme in the View window again.

Under the Theme pull-down menu choose Convert to Grid. A Conversion Extent window will appear. Set the Output Grid Extent to the Same as Mask.shp. Here we have set the Output Grid Cell Size to 5 feet. Be sure to hit return to update the number of rows and columns when you change the output grid cell size.

Next, a Conversion Field window will appear. For this example, the new field created in the step above (called Value) is the conversion field. Click OK. Name the grid in the next window. There is no need to join the feature attributes, so click No in the next window. Add the new grid to the view as a theme. Your grid is ready to use as an edit mask.

Setting the Analysis Properties

Under the Analysis pull-down menu, choose Properties. This window is important and allows you to set the analysis cell size. The recommended cell size is 2 meters or 5 to 6 feet, depending on map units and projection. The cell size affects the resolution of the interpolated surface. Using a cell size smaller than 5 feet increases the "noise" associated with the resulting contours, while a cell size greater than 5 feet reduces the resolution significantly. Using a cell size of approximately 5 feet (or 2 meters) will result in relatively smooth contours while retaining resolution appropriate to the nature of the LIDAR beach mapping data. The Analysis Extent can be set the same as the edit mask (Mask.shp). For Analysis Cell Size, choose As Specified Below, then enter the appropriate cell size below. For Analysis Mask choose the Edit Mask Grid (Maskgrid). Click OK.

Creating a Grid Using the Edit Mask

Activate the point shapefile (Sullivan.shp). In Spatial Analyst v.1.0, under the Analysis pull-down menu, choose Interpolate Surface. The directions are slightly different for Spatial Analyst v.1.1. Under the Surface pull-down menu, choose Interpolate Grid (if you do not plan to use the interpolated grid for any other analysis, you may want to choose Create Contours at this step.) The recommended method for interpolating the surface is inverse-distance weighting (IDW) using the default number of Nearest Neighbors (12), changing power to 1. Choose Elevation as the Z Value Field. Click OK. Processing time will depend on the number of points and the amount of memory available in your computer. The process may stop if you are working with more data than your system can handle. The interpolated surface will be added automatically to the project when the processing is complete. Note: To reduce processing time choose Fixed Radius instead of Nearest Neighbor. However, a small search radius may result in no data values within the grid.

Creating Contours

To create contours, activate the new grid theme, Surface of Sullivan.shp. Under the Analysis pull-down menu, choose Create Contours. You will find this option under the Surface pull-down menu in Spatial Analyst v.1.1.) Choose a contour interval of 1 or 2 feet and base contour. Click OK. The Legend Editor may be used to add graduated color to the contours. For more information about the Legend Editor, click here.

Computing Hillshades

Additional analyses and graphic display, such as hillshading, may be performed using the interpolated surface grid of sullivan.shp. To compute hillshade, activate the grid theme. Choose Compute Hillshade under the Analysis pull-down menu. You will find this option under the Surface pull-down menu in Spatial Analyst v.1.1. A window will appear to adjust the azimuth and altitude. Click OK to accept the default parameters.

Using the new hillshade as the brightness theme can add dimension to the color-coded elevation grid. Double click on the grid theme to activate the legend editor. Click on the Advanced ... button. In the pop-up window, choose the hillshade theme as the brightness theme. Click OK then Apply. Experiment with the brightness values in the pop-up window to achieve the desired effects.