The GeoInformatics Training Research Education and Extension (GeoTREE Center), in cooperation with the Iowa Department of Natural Resources, has developed additional functionality for their GeoTREE LiDAR Download web mapping application. The new functionality allows the user to draw a rectangular study area for an area they are zoomed into. After drawing this rectangle the user will be prompted to decide which derived products they would like (DEM, Hillshade, Contour) and for their email address. After submitting this information the user will receive an email with a link for each product they chose to derive. The emails could take anywhere from a few minutes to more than an hour depending on the size of the study area chosen. The derived products are being processed based on the bare earth ASCII (.xyzi) LiDAR tiles.
This new functionality is intended for use for study areas that are approximately 50 tiles or less in size (~200 sq. km).
The DEM and Hillshade products will be in the form of .img files while the contours will be a zipped file containing a Shapefile. The DEM and Hillshade products have a resolution of 2 m while the contours are in 2 foot increments. The elevation value of the contour is stored in the column labeled feet in the shapefile.
Below are the general steps to utilize the new functionality.
Zoom to an area of the state that represents your study area.
Click on the ‘Draw a polygon to select LiDAR tiles for derived products’ tool.
Drag a rectangle for your study area.
When the new page pops up, choose the products you would like, enter your email address, and click
Submit. Close the page when you are finished.
Check your email periodically for an email for each product.
When you receive the email for each product click on the link to download to the directory of your choice, unzip, and rename products as you wish.
Several open source geoprocessing techniques are used to process the original Iowa .xyzi bare earth LiDAR data into derived products including the Points2Grid utility and the Geospatial Data Abstraction Library (GDAL). The Points2Grid Utility generates Digital Elevation Models (DEM) using a local gridding method on point cloud data. The Points2Grid utility is supported by the Open Topography Facility. The GDAL library is used to fill no data cells in the DEM, to attach spatial reference information, and to create the hillshade and contours from the DEM.
The GDAL library fillnodata command uses an interpolation method to fill in cells of the DEM where there are no data. The gdal_translate command is used to assign NAD 83 UTM Zone 15N spatial reference (EPSG:26915). The gdaldem hillshade command is used to produce the hillshade with default parameters chosen. The gdal_contour command is used to produce contour Shapefile with a 2 foot (0.6096 m) contour interval. The 'feet' field holds the elevation value.
These products are derived through open source utilities and do not follow the exact process that was used to create the 3 m resolution DEMs and the 2 foot contour vector Shapefiles distributed by the Iowa Department of Natural Resources. Although the products will be very similar they will not be identical.
No attempts have been made to estimate vertical or horizontal accuracies for these products. The suitability for technical, scientific, or finished cartographic purposes is unknown and should not be assumed without testing.