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I'm not surprised that you haven't got an answer to this seemingly simple question yet. You've opened a whole can of worms here! Take this DEM and vector streams layer as an example: At the simplest level, you could always perform a profile analysis with your streams and see whether they have predominantly downward profiles: Consider for a moment what ...


3

It may seem like laziness on the part of Watershed tool developers to stick with the simplest and oldest flow algorithm, D8, but there is a very sound reason for doing so. The difference between the D8/Rho8 flow algorithm and the more advanced algorithms that you mention (e.g. D-infinity) is mainly in their inability to represent the dispersion of overland ...


1

1) I would use the reclassify tool. Set it to twenty 'equal interval' breaks of 5 each. 2) Then use the raster (created in step 1) for rendering. Bring it into ArcGIS and select 20 classes. You could certainly skip step one but it may be nice to have this layer if you plan to use it in future analysis. You could also just do 2 and save as a layer file and ...


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Alpheus, I'm not sure if you've already solved your problem or not but I would recommend against converting your DEM to a 16-bit raster. This is very likely to reduce the precision with which the elevation data are stored and can be very disruptive for any subsequent analysis, e.g. flowpath modelling or slope analysis. If your data are currently floating ...


0

If you have the GDAL plugin installed, you can use the Clip Raster by Mask Layer tool to clip your DEM to a vector polygon: If this is the tool that you are trying to use already, then is there some kind of error that is being reported when you run the tool? Knowing what that is may help to diagnose the problem.


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Building on Michael's excellent answer, I would recommend using the Con (Spatial Analyst) tool to take a "slice" out of your DEM. The first screenshot shows the parameters you would likely want to use. The second screenshot shows the results of the Con function (as stylized MDOW hillshades) derived from the resulting DEM's.


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There is two ways of doing this, the best is with Spatial Analyst using Extract by Attributes using the query value < 1.5, this will put NODATA in the areas where the height is greater than 1.5m. If you want to cap the raster to 1.5 so that the values are 1.5 where the raster exceeds 1.5 then use con: Con(InRaster,InRaster,1.5,"value <= 1.5") will ...


1

I have had good luck with FUSION's (manual here) GroundFilter command. I've had no problem handling 40 million points (unclassified), so wouldn't expect an issue with 100 million.


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When you perform a flow accumulation on a DEM, the upslope area is defined as all of the inflowing grid cells AND the cell itself. Thus, a grid cell is actually part of its own contributing area. The minimum upslope area (not specific contributing area) is equal to the area of one grid cell, i.e. a grid cell with no inflow cells. I think this is the reason ...


5

Martin is correct that while your workflow will do well for a specific user case, it doesn't account for many of the issues that road embankments create for flowpath modelling using fine-resolution LiDAR data, such as the problems with discontinuous flow in roadside ditches and the effects of minor unmapped culverts (which can alter flowpaths considerably). ...


1

You might as well make a DEM/TIN (using ArcGIS) of the shapefile you have. That will give you the DEM you need to convert to GMT format. After a quick Google search, I found a couple of links.. convert SRTM30 data to grd Shapefile to GMT DEM file Also check the this PDF, mostly after the 15th Page - GMT and Gridded Datasets There is a clear cut ...


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I realize that this question was asked quite some time ago but there was no other answer provided so I thought I'd add something. The drainage network can be derived from a digital elevation model (DEM) using a threshold flow-accumulation raster. Please take a look at my answer to this more recent question for details on that process. As for extracting a ...


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The answer will depend on the requirements of your specific workflow and application but I can offer you advise on how a drainage network is generally extracted from a digital elevation model (DEM). The key to extracting a drainage network from a DEM is creating a flow accumulation raster, i.e. a raster for which each grid cell contains a value that is ...


5

This is a good question, and one that I tend to get asked from time to time. First, as you've pointed out, the equation for TWI = ln(a / tan(B)), where a is the 'specific' catchment area (i.e. the upslope inflowing area normalized for a measure of contour length) and B is the slope gradient, in radians, at the grid cell. As you correctly pointed out TWI will ...


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"CELL" type means integer type (see also http://grasswiki.osgeo.org/wiki/GRASS_raster_semantics#Raster_map_precision_types). Since SRTM is delivered as integer map but i.topo.corr expects a floating point map, you need to convert/resample that map beforehand. For resampling methods available in GRASS GIS, see the manual or Wiki, especially: ...


3

Chris, Maksim is right, you should be able to solve this problem by mosaicing each of your individual tiles into a single DEM. The problem that you have currently is that each of the individual DEMs has it's own display minimum and maximum values over which the same greyscale palette is being stretched. Consider, for one tile the 256 (perhaps more) grey ...


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Creating a mosaic dataset and adding in all your rasters should solve your problem. Create Mosaic Dataset (Data Management) Add Rasters To Mosaic Dataset (Data Management)


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An option to normalize lidar clouds (and keep it as a point cloud) is Fusion. One will need the command line ClipData together with the switches dtm:file, which is the bare-earth model (DTM) + height. ClipData description says: ...When used in conjunction with a bare-earth surface model, this logic allows for sampling a range of heights above ground ...


2

There are a number of tools in QGIS for generating contours, though most require a raster input rather than vector points. As such, your best bet is to use the Contour Lines From Points tool: Models -> [Example Models] -> Contour lines from points You have to set up the SAGA plugin. You can do it based on the documentation: QGIS Configuring external ...


3

I was a bit surprised that no one had answered this question yet so I decided to look at how you would perform the task of extracting a watershed from a digitized point along a stream network in QGIS myself. I found that there are about five tools in QGIS for watershed extraction among the various toolboxes. Unfortunately, and perhaps a QGIS expert here can ...


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I've had to map ditches from 1 m LiDAR derived DEMs of agricultural landscapes before. It's certainly a challenging task to come up with a workflow that is suitable. You're ability to successfully extract a ditch network will depend on a number of factors. For example, are you only interested in roadside ditches? If so, are the roads on embankments (as is ...


5

I've been in LiDAR processing for a couple of years now. The best approach we've found is to classify the suspect water points to something other than ground. Should be easy just classifying based on intensity (near nadir points will have high intensity, whereas turbid water will be close to 0) and laser shots are usually absorbed near shore anyway. ...


2

This is a common ArcGIS problem. I believe as you zoom-in some of the areas that seem not to be there begin appearing as if it was a rendering issue... I think, although it has never been confirmed, that it most often happens when mosaicing tiles from File Geodatabase (FGDB) sources and outputting the mosaic raster to the same or other File Godatabase. ...


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Usual thing for this tool. Try replacing it by few mosaicing, say working in 'columns', i.e. repeat it 6 times plus one


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Create a mosaic dataset, right-click your new dataset and choose "Add Rasters", navigate to your folder containing your rasters, click OK.


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Fusion LiDAR can do that, try download. User manual is simple and you can process lidar files via command line. http://forsys.cfr.washington.edu/fusion/fusionlatest.html edit: FUSION Manual: http://forsys.cfr.washington.edu/fusion/FUSION_manual.pdf try to search it using "normalize". You can create bare earth file using GroundFilter.


3

This is the exact problem that I am currently working on for my own research. As a result I have been working on a plugin tool for the open-source GIS Whitebox Geospatial Analysis Tools, called 'Isolate Ground Points'. It will take an input LAS file or point-type shapefile and output only the ground points in a multipoint-type shapefile. It turns out that ...


2

you can use the SRTM DEM as input and create the color relief + hillshade with gdaldem. If the file is too big for your purpose, you can also use ETOPO1.They also provide colored images on this site. If you are looking for a portal, you can visualize several DEM here


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You may check out the 'Trace Downslope Flowpaths' tool in Whitebox GAT. It will identify the cells in a DEM that receive flow from upslope target cells. It is however based on the D8 flow algorithm and therefore cannot model flow dispersion, which would yield downslope 'areas' as you are referring to them as. Nonetheless, there is some debate in the ...


4

Landsat and Modis are optical sensors, which means that they provide digital numbers of reflected materials that are within the electromagnetic spectrum. These values correspond to the wave length of the corresponding satellite band. To get elevation from just the raw values would be impossible. The only potential means to collect elevation information would ...


1

If I understand the question correctly, the QGIS Raster Calculator will do what you want. For some examples that touch on your problem see: http://spatialgalaxy.net/2012/01/25/using-the-qgis-raster-calculator/ I think you would then clip the output raster using your flood polygon. N.



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