Contours reveal the relief or topography of a region. LIDAR data may be used to create detailed topographic beach maps. Before you can create topographic maps, you will need to acquire a LIDAR data set. These data sets can be extracted from 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 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 (processor, disk space, amount of RAM, etc.), 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 1Mb is recommended for this test of your system.

Creating an Edit Mask

An Edit Mask can also be used to limit the interpolation area to the beach face. This is important because the 1997 and 1998 LIDAR data sets extend inland and include 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 that 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 on Yes to save the new polygon.  This image shows the new polygon theme. 

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 Shapefile Attribute Button to open the table of active theme. Click on the attribute table. Under the Table pull-down menu, choose Start Editing. Under Edit, choose Add a 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 yellow. Click on the Edit Table button. This button allows you to change cell values. Click on the cell under the new field. It should have a value of zero to begin. Change the value to 1 (one). 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. Remember to hit return to update the number of rows and columns when you change the 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 data. The Analysis Extent can be set the same as the Edit Mask. For Analysis Cell Size, choose As Specified Below, then enter the appropriate cell size below. For Analysis Mask choose the Edit Mask. Click OK. 

Creating a Grid Using the Edit Mask

Activate the point shapefile. In Spatial Analyst v.1.0, under the Analysis pull-down menu, choose Interpolate Surface. (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 weighted (IDW) using the default number of Nearest Neighbors (12) with a power of 1 (one). 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. 

The directions are slightly different for Spatial Analyst v.1.1. Under the Surface pull-down menu, choose Interpolate Grid. Follow the instructions above.

Note: To reduce processing time use Fixed Radius instead of Nearest  Neighbor. However, a small search radius may result in No Data values within the grid.

Below is the resulting grid from the interpolation process using an edit mask.  A custom legend was added to the grid after it was created.  

Creating Contours

Computing Hillshades

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.