I have data on different populations and I need to get the elevation values for each population from the main river in the study area. I have 3 datasets: populations (polygon shp), dem (raster), river (line shp). Using "zonal statistics as table", I extracted elevation values from sea level. However, I am interested in getting elevation values relative to the main river. Any suggestions?
Usually a raster data set is developed for elevation based on topigraphic maps that have elevation points referenced. From that, a digital elevation model (DEM) is developed based on 30m or 10m pixels. However you may want more accuracy, so our paper on researchgate from ~2007 suggests substantial improvement with LiDAR. Under clear water conditions, green LiDAR can estimate channel depths and variations. Most normal LIDAR can give detail of channel bank locations, but the light pulses are absorbed in water. For detail stream surveys of channel dimension, total station surveys are typically done. Alot will depend on your intent and need for detail. I am not a GIS expert, but use ARCMAP intermittently. The topographic coverages are typically rectified to elevation above sea leval. Because many streams are relatively narrow, the 30m DEMs are unlikely to give detail desired for project work, but may be good enough if only an approximation or relative elevation between a group of sampling points is desired. There is a map of approximate stream elevations of Indian Creek Hydrologic Analysis that used the basic GIS DEM, but you will see on the linear profile bumps in channel due to data imperfections or perhaps some effects of channel depth changes, roads, beaver dams, large logs perhaps, etc.
Hi Rocio,
I would do as follow:
- Find a way to get the lowest point on your river. Could be the outlet, but not necessarily. To do so, you can either convert the polyline to points and extract per point, or convert it to raster and extract per mask. Either will give you a set of elevation pixels that you can browse to find the lowest value.
- Then you can use the raster calculator (or the subtract tool) to subtract the lowest elevation to the DEM. This way, the lowest elevation in the river will become 0, and the rest of the elevation values will be set according to it.
Make sense?
Thanks Fracois!! It makes a lot of sense!!!! I really appreaciate your feedback, as it is helping me a lot!
Rocio, if you are still interested in developing a relative elevation model from an existing DTM there are several tools that will work with ArcGIS to do an adequate job. The River Bathymetry Tool ( https://essa.com/explore-essa/tools/river-bathymetry-toolkit-rbt/ ) and Geomorphic Grade Line tool ( https://github.com/helstab/GGLREM ) are both very usefull and function well.
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