QGIS Terrain Analysis: hillshade, slope, aspect (Version 3.x) 

hey i have a doubt, im trying to render slope with qgis for a dem of a part of a map,a district basically, but idk why when i do so, it comes like black and white with no actual definition

You're welcome!

Glad it was helpful!

You can use the Python console to input the specific coordinates and return the value of a raster. This video shows you how: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-aemGjVwBGzQ.html

I haven't done any configuration. Depending on your DEM metadata you may have to configure some things in symbology. Some DEMs just have course spatial resolution.

Thanks! You could use the raster calculator to multiply S-W by 1 and everything else by zero. That would give you a new raster with zero values in areas that aren't S-W and aspect values for S-W slopes

@@geospatialschool Thank you!

Yes it is possible. You should look at my videos about the raster calculator. That will help you determine the areas that meet your criteria. Then you just need to count the number of raster cells and multiply by cell area.

@@geospatialschool thank you again!

This is a USGS 1m DEM you can get the from the US national map website

Up plz

You probably just need to adjust your min and max values in the symbology tab

I can try to get a raster calculator tutorial out soon.

Check out this video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-plHH-BwSnDQ.html Hope it helps!

Nothing has been done to modify the DEM. Typically, you only full a DEM for hydrologic analyses

@@geospatialschool Right on man! Thanks again for your work. Nice condensed refesher.

Are asking how to compute erosion risk? How to find an erosion risk layer? Or how to display a layer of erosion risk that you already have?

Not sure what you mean. Can you explain?

I'm not exactly sure what your problem is?

@@geospatialschool when I ran the slope tool the output only shows areas that are flat. The output raster has values of 0 or 89.

Not sure why that is happening. Have you tried both the QGIS and GDAL algorithms? Have you tried a different DEM or changing the algorithm parameters to see if that produces different results? I've done a little research and it's possible that if your DEM is in a geographic projection (degrees) you may need to convert it to a projected coordinate system in the same units as your elevation values (e.g. meters) for it to work. See these posts: gis.stackexchange.com/questions/143139/getting-incorrect-slope-values-from-an-aster-dem-in-qgis gis.stackexchange.com/questions/290190/issues-with-qgis-slope-analysis

@@geospatialschool Thansk. If I need more help I will post on there.

Can you be more specific? What type of data (raster/vector, continuous/discrete, etc)? How do you define "straight"?

Open Source Options Thanks for replying. I did some more searching and found some different answers. In most cases the data is raster. The lineament analysis can be done with other data structures as well. Straight lines are relatively straight. That is one eye sees a ridge’s crest as straight for some length and then a break and maybe continues offset or on a new azimuth. Similarly a water course may have segments that are straight. Curved lines that loop or repeat can are also used in the analysis for structures in the geology. Man made items often mimic and follow the subtle changes in the fabric of rock type, faulting and other boundaries. I use image filters to accentuate features and my eye to identify the pertinent shapes. My best hope was to find an automated system do what my artistic eye does.

Open Source Options this video gets to the why and what. Start at 3:30 seconds to jumpstart the Lineament section. Later he touches on Eigenvalues very briefly. I am thinking the advancement in neural network systems and python tools will have some open source opportunities for this kind of technique to do spatial geopetal fabric analysis. Any ideas?

Can you send the video link?

Open Source Options ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Z-tn-BCKauI.html

If you open QGIS with GRASS, there is a raster reclassify tool in the GRASS algorithms that can reclass the slope raster.

It's either something with the input DEM or the parameters you set for the hillshade. First, make sure you used and elevation input for the hillshade tool and not a raster representing something else.

Mine is rough too, maybe because I am using a 90 m DEM and not a 30 m like in the video. Check for you. The hillshade still render something usable, the aspect is barely readable and the slope produce a no sense raster (almost all pixel with the 90° slope value). Can slope analysis be run on a 90m model ? I work in West Africa and I dont think I can get a more precise model