DEM

#Exporting #GIS #attribute #table from a #raster (#DEM)

Here’s a video on how you can export the attribute table from a raster DEM.

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#Joining #GIS #digital #elevation #models #tiles

#Mosaic tool, the lifesaver tool for combining multiple #DEMs.

Back in my undergraduate engineering days, I struggled with unifying multiple DEMs–especially when my “study area” had a spread over multiple DEM tiles.  In this video, you’ll see how to combat this dilemma.  This video was made using the amazing and freely available data from Geogratis.

Now, when using the mosaic tool, make sure you do some due diligence.  I always advocate checking your projection.  That means that if you are mosaic-ing a small area, try converting your data to a planar projection (like UTM).

Another tip is to break up the mosaic process.  This means that if you have 20 tiles to mosaic, try doing two to three at a time instead of all at once.

Lastly, if you are finding the processing is slow, try changing your default ArcMap 10 Background Processing to Foreground Processing (click here to see my WordPress posting).  You will find this processing will make your ArcMap 10 experience very much like ArcMap 9.3 with the pop-up progress.  Personally, I like this pop up because I can see the processes and helps optimize my experience in ArcMap — rather than overtaxing the program.

How to #delineate a #watershed #GIS

How do you #delineate a #watershed using a #DEM.

One of the fun parts of sharing knowledge on #YouTube is entering uncharted waters.  While sharing GIS videos on YouTube, I realized there were insufficient videos to show how to delineate a watershed from scratch using ESRI’s ArcMap.  Actually on a basic search on YouTube, there was nothing.  While going to school is great, there has to be something online that can serve as a quick refreshers for students and professionals alike.

When undertaking this fun challenge, it was a nice refresher on the classic was to do this–especially when I had been delineating watersheds using hydrological model tools.  Reviewing the video, yea its a little rough on the edges, but it represents those fundamental steps in building the layers for delineating a watershed.

To start the watershed delineation process, we first start with the Mosaic step (click here for a video refresher).  After essentially joining raster layers, we start with filling the sinks and determining the flow direction.

 

After the mosaic, sink filling and flow direction determination, we started the flow accumulation and digitizing pour points.  In this video, we recieved great questions from +Josh McGee and +Gladys Chelangat who asked about the criteria for breaking the classification with “high flow accumulation” in the video comments.  We break the classes to define the difference between high flow and minimal flow.  The flow in channel will be significantly higher than those overland flow paths.

 

In the final video, we merged the watershed delineation steps to bring the components together.  We received a fundamental and important comment from Nototo on how important it was to have the same raster cells.  I was very happy to see a fellow GIS-er work through a problem.  As with any data you work with, it is important to check the project and in the case of raster with mosaic operations, you want to make sure all cells are the same resolution.

 

Did you find what you were looking for?  Feel free to share your appreciation in the comments of this blog, on Google+ (teachugis) or on Twitter (www.twitter.com/teachugis).  As always, thanks for watching. 🙂

 

 

 

#Creating #cross #sections with a #DEM #GIS

Creating #CrossSections with #DEMs is a fun way to review the the change in vertical elevation with respect to distance.

In this video we defined a line segment and produced a profile graph using ArcMap 10.  This is an awesome way to do preliminary ground truthing.

To do this video, we attained the DEM from GeoBase and clipped it using a Toronto Regional Boundary from the Toronto Open Data.  We applied segments along channel cross sections.

After posting this video on cross sections, we had amazing questions from viewers on how to expand the usability of this output.

Like +NachoValdes7 who asked an excellent question on calculation the area of the cross section.  The difficulty with this tool is that while you can customize the axis and titles, the output is graphical.  Unfortunately this tool leaves most users with the difficult task of gridding the output and counting the cells then multiplying the number of cells with the true dimension.

i.e. each cell is 100m x 100m.  You have 30 cells
Area = 30 x (100m x 100m) = ____m2

Another fun question was from +Diarmuid Duggan who asked if you could define those line segments used to derive the cross sections using an existing polyline.  So that was a very fun and out-of-the-box question.  Here’s that video (part 1, 2 and 3).

As always, there’s multiple ways to arrive to the same solution.  If you have a solution that’s equivocally interesting, feel free to screen capture it on YouTube and share. 🙂

Did you find what you were looking for?  Feel free to share your appreciation in the comments of this blog, on Google+ (teachugis) or on Twitter (www.twitter.com/teachugis).  As always, thanks for watching. 🙂