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I need to estimate the elevation at various access points of some rivers in South Carolina. I use the elevation information to calculate the average gradient (feet/mile) between two access points. The gradient between points A and B on a river is equal to the elevation difference between A and B divided by the distance along the river between A and B.

Usually I get the elevation information from contours on the USGS topo maps. However many South Carolina topo maps have only 20-foot contours when they reach the lower Coastal Plain and within about 20 miles of the coastline. Most states have topo maps with 5-foot contours near the coast.

I learned that South Carolina has been working on acquiring accurate elevation data for the entire state. Digital Elevation Models (DEMs) are available for many counties. The data is available here: http://www.dnr.sc.gov/GIS/lidar.html

They mentioned I would need some kind on software to read the DEM data and draw the contours with. They suggested that I try Global Mapper. I tried the trial version but got the following error: “ Unable to load overlay gdb -- This release of the GeoDatabase is either invalid or out of date. The ESRI File Geodatabase library does not support GDB files from ArcGIS v9.x or earlier. You will need to convert your GDB data set to a newer version using ArcGIS v10.x or later to make it readable.”

Next I looked at ESRI’s ArcGIS Explorer Desktop. I tried to download the trial version, but the larger file was corrupt twice, so I requested a disk be mailed to me.

My question to this group -- is there another program available to allow me to see the DEM contours? I do not want to pay for an expensive program considering I need relatively little from it.

I would appreciate any advice from users with experience in this area.

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2 Answers

I would recommend using QGIS, using GDAL Tools to create contours (free), see link below.

http://planet.qgis.org/planet/tag/dem/

You can also use QGIS to create other raster terrain layers (e.g. shadded relief, slope, aspect...etc), see link below.

http://www.gistutor.com/quantum-gis/20-intermediate-quantum-gis-tutorials/48-quantum-gis-qgis-raster-based-terrain-analysis-techniques.html

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Quick Terrain Reader from Johns Hopkins APL will allow you to visualize the LAS data from that site, and may let you measure along profiles. They even offer a free trial of their full Quick Terrain Modeler software, which will let you create DEMs from and make measurements from the LAS files. The Reader is free, and the Modeler is fairly inexpensive. In my previous job I was a customer/user of Modeler, but that was three years ago.

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