Creating Grids Using ARC/INFO®

LIDAR data are unique in that elevation measurements are collected in very high density. The close spacing of many elevation points allows generation of high resolution elevation maps. Other data collection techniques, such as profile surveys of the beach face or randomly spaced global positioning system (GPS) elevation measurements, typically do not produce as much detail. It is generally not possible to collect as much data with a ground survey as with a LIDAR survey.

The high density of the data creates large files, especially when large geographic areas, such as an island, are examined. Elements of computer hardware, such as RAM, central processing unit (CPU), and hard drive space, play a critical role in the user's ability to process LIDAR data sets. Without adequate hardware resources, it may be difficult to process data for large spatial regions. Additionally, many software packages may also have difficulty processing LIDAR data sets. ARC/INFO is one example of a powerful GIS software package that can process large LIDAR data sets. The contours and grids provided with this CD-ROM were all generated with ARC/INFO's GRID module. The following tutorial demonstrates the steps taken to create those data products. It is helpful to know ARC/INFO basics before beginning this tutorial.

Before you can create grids or elevation maps in ARC/INFO, you will need to acquire a LIDAR data set and import it into an ArcView® project file. 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 of LIDAR data. For more information about the BeachMapper application, click here. The other source from which to acquire 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 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 your computer system specifications (e.g., processor, disk space, amount of RAM), processing time will vary. 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.

Converting a Shapefile to an ARC/INFO Coverage

To work with LIDAR elevation data in ARC/INFO, you must convert an ArcView shapefile to an ARC/INFO coverage. If you need instruction on converting the raw text file to an ArcView shapefile, click here. Converting the raw text file to a shapefile first maintains all attributes, in this case the x,y,z values. In ARC/INFO it is then a simple step to convert the shapefile to a coverage. At the Arc prompt, type the command SHAPEARC followed by the shapefile name and the name of the new coverage to be created, followed by the word point for the out_subclass.




The next step is to "build" the coverage. Once the coverage is built, you will be ready to work in the GRID module.

Generating a Grid in ARC/INFO's GRID Module

Initiate the GRID module by typing grid at the Arc prompt. A screen will appear that may be minimized. At the Grid prompt you must first set the window. In essence this step tells ARC/INFO where you are in space. You can use the point coverage for this step. For example, the command would look like this:

Grid: setwindow edisto

Now you can process the grid with the following command:

This method employs inverse distance weighted interpolation using six nearest neighbors and a cell size of 5 feet. The search radius was limited to 50 feet. As the process runs, the progress is monitored and updated on the screen. This will give you an idea of how long the entire process will take. Once complete, exit the GRID module by typing quit at the Grid prompt.

It may take minutes to hours to run this process, depending on the number of points in the original coverage. It may be wise to begin the process and allow it to work overnight. You can view the grid in ArcView using the Spatial Analyst® extension.