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14

You are correct that distortions in the projection can bias flow direction (and flow accumulation) estimates. (Using "unprojected" data is tantamount to using the highly distorting Plate Carree projection.) For merely delineating basins, though, there actually is little problem: although the flow directions and flow amounts will be wrong, the projection ...


12

Given that you have the LIDAR DEM, you should use the streams derived from it. That guarantees perfect registration. The crux of the idea is to estimate mean slopes in terms of the elevations at the ends of the segments. One of the easiest procedures is to "explode" the stream network into its component unbranched arcs. Convert the collection into a ...


12

Convert your stream vectors to raster with a value of 1 and the same extent and cellsize as your DEM. In the Raster Calculator use a map algebra expression something along the lines of: Con("rivers"==1, "DEM" - 1, "DEM") If you want to burn in the streams more than 1 elevation unit, change "DEM" - 1 to "DEM" - a bigger value. To implement the Whitebox ...


9

First, you need to calculate at least the slope. F.ex I have the following data: Then put the correct data as variables to the module: And at last you should get the result: UPDATE With Catchment Area as input the results are:


7

Whitebox GAT (open-source hydrology and remote sensing package) has a method by this name in its Hydrology utilities. Whitebox is unique in that it exposes the source code and algorithms used by the analysis via the UI (note the View Code button). Even if you intend to isolate your procedures to ArcGIS, there may be some benefits to experimenting with ...


6

The analysis has already been done in a reply to the antecedent question, but perhaps an illustration will help. There are two major components of error: the "d8" algorithm, which represents flows in only eight cardinal directions, and the effect of projection (or lack of it). Let's focus on the latter, because this seems to be the principal concern. The ...


3

Tapering will emerge from an appropriate kernel smooth of a transformed streamflow grid. For a start, consider using the square root of stream flow and use an exponential kernel: the bandwidth of the kernel will determine the apparent width on the map. Here is a sample workflow using operations commonly found in raster GISes such as Spatial Analyst, ...


3

The slope is obtained by extracting the elevations at the sample point and the point 100 meters upgradient along the main stream (and then dividing their difference by 100). Consider these criteria for finding the upstream point: It must lie on a stream. It must be upstream of the sample point. The stream distance to the sample point must be 100 meters. ...


3

I can't speak much on GRASS specifically, but it looks like what you're trying to do here is develop flow direction and accumulation grids - I'm pretty sure GRASS already has that functionality built-in. Once you've developed those grids, you can use a regression equation (the USGS develops these: http://water.usgs.gov/osw/programs/nss/pubs.html#wv) to ...


3

You could perform a watershed analysis yourself, but its a time consuming process. One good source of existing river catchment data is the HydroSHEDS project, which provides high resolution basins for much of the globe, including the UK.


3

The trick is to Google "UK river catchment map". (Other combinations that include "watershed" don't work.) You will find River basin management plan documents. River basin district maps (downloadable files). An interactive map. Maps by the UK Environment Agency. Scottish river basin maps and data. A search involving "watershed" did turn up a historical ...


2

I'm afraid you are lost with QGIS. According to the Readme.doc, the data is in Arcgis 9.3 format. QGIS and GDAL only support gdb file databases written by Arcgis 10 and above. As an alternative, you can download the data from NHD Plus in other formats too. These are shapefiles and Arcinfo hdr raster files, depending on the topic. Both formats can be read ...


2

Yep, turns out I wasn't honoring the wisdom of the beginner's mind and I jumped right to an assumption that my problem was way more complex than simply not having run the Set Flow Direction tool. Entries in the attribute table of the NHDFlowline file suggest that "WITH_DIGITIZED_DIRECTION" is the right parameter to go with - and it seems to check out ...


2

If you don't find a map, SAGA GIS has a module called watershed basins that uses a dem and the channel network (your rivers). If you use a coarse dem this goes fast. http://www.saga-gis.org/saga_modules_doc/ta_channels/index.html


2

Recompute the flow accumulation by setting the input ("rainfall area") to 1 within the cells of a given land use and 0 elsewhere. Extract the values at the stations (one operation via the "extract values to points" tool). Multiply these results by the squared cellsize to estimate the areas. Repeat for each land type. Join the results on the station ...


1

I am doing some Hydrology analysis on my end and as I was to create my Flow Direction raster I remembered your post. This is just a stab in the dark but, in ArcGIS 10 there is an option to create an output drop raster. I wonder if it somehow could be used to solve your problem. The drop raster shows the ratio of the maximum change in elevation from ...


1

So through discussions on another forum I was able to figure out why the standalone script was running so terribly slow. This is because I did not have the gp.ScratchWorkspace property set. Once I set that to mirror my ArcToolBox General Environment setting using the same directory, I was able to run the Python script from the command line in approx. 6 ...


1

Your question is poorly stated. It as at once general, as it asks if anyone has ideas about how to implement MFDs, and specific, as it seems to ask what is wrong with you code. I'll try to answer this second conceptualization of your question here. You claim that the code you've posted is not optimized. I have rewritten the code below as I think you should ...


1

It wasn't clear from the description of your project whether you are trying to model the passage of a flood wave through a 2-D flow grid (i.e., the change in inundation area as a function of time as the flood passes from upstream to downstream), or whether your output is to show the highest flood elevation at any given point caused by a single flooding event ...



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