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I'm working through the QGIS Training Manual where I have a step to get ...

... raster images (SRTM DEM) which cover the region you have selected for your course.

SRTM DEM can be downloaded from the CGIAR-CGI: http://srtm.csi.cgiar.org/

You’ll need images which cover the entire region you have chosen to use.

It then shows an example image (Image 1) for Swellendam, Cape Town:

enter image description here

However, the only image I can find for my region (UK) on CGIAR-CGI is similar to the image shown below (Image 2), which only seems to show digital elevation and not any other detail such as houses, roads or fields like 'Image 1'.

enter image description here

Question:

Does a SRTM DEM raster image exist for the UK region on CGIAR-CGI that has the same level of detail as 'Image 1'?

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    Short answer: NO. SRTM has a fairly coarse resolution but is fantastic for being standard world-wide. If you want a better DEM contact your local government mapping section and see if they've got a LiDAR dataset (fees and charges may apply)... in the 1st world almost all cities/suburban areas have some sort of LiDAR coverage - of differing age/resolution. Image 1 is not SRTM DEM it's either satellite imagery or orthophotography. If you want an image like that there is an Open Layers plugin for QGIS that allows you to view Google (but not print it).
    – Michael Stimson
    Jun 3, 2015 at 21:49
  • Ah, so the example image, 'Image 1' isn't an SRTM image?
    – Chris Snow
    Jun 3, 2015 at 21:52
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    SRTM stands for Shuttle Radar Topography Mission. The purpose was to develop worldwide elevation data. The only "images" would be hillshades generated from the elevations (there was no camera involved, so no pictures; you'll need to provide your own).
    – Vince
    Jun 3, 2015 at 22:04
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    Definitely not. SRTM is 32bit elevation data - no colours (except those you apply yourself if you wish). This is the plugin I was referring to: plugins.qgis.org/plugins/openlayers_plugin - when you go to print the layer turns off (as per license agreement with the providers) so you can see and digitize off the image but can't export/print it.
    – Michael Stimson
    Jun 3, 2015 at 22:04
  • The DTED rasters are 16-bit (not 32-bit). I'm pretty sure the non-DTED is also 16-bit (signed).
    – Vince
    Jun 3, 2015 at 22:07

2 Answers 2

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Chris,

Your confusion stems from the distinction between the image represented by image 1 (a true color image) and the DEM you downloaded. The two are very different things and this distinction is one of the very great things about raster analysis in geospatial science!

Image 1 is a high-resolution true color satellite image or aerial photograph such as you would have created in Lesson 1.2. Image 1 is similar to what you are used to -- that is, rasters represent things similarly to how we see them with our eyes. In that data, each cell has three values associated with the amount of light reflected in (roughly) the same wavelengths our eyes see. However, as I intimated before, the cool thing about rasters is that you can put ANYTHING in those cells -- they don't have to represent the color we see on the ground. They could represent the elevation, the amount of water in the atmosphere, the forage quality for a migrating bird, the sea temperature, the wind vorticity... get the idea?

The SRTM DEM you are reading about in the tutorial is a type of elevation product with a spatial resolution of 90m. DEM, or Digitial Elevation Model, is a type of raster data in which each cell value corresponds with the elevation of the area represented by the cell. SRTM refers to the fact that these data were collected by the Shuttle Radar Topography Mission.

To answer your questions specifically (they were changed while I was composing this response)

1) This question was deleted, but said "Are there STRM DEMs for UK? " A: Yes, and it looks like you may have found it. 90m resolution is not bad if you're interested in broad landscape analysis like watershed modeling but you aren't going to use it to find the slope of a football pitch. For that you would need a higher quality aerial-generated product like LiDAR. Generally you aren't going to get DEM resolution as fine as your imagery.

2) You can get IMAGERY for UK. You'll want to get familiar with WMS, or Web Map Services, an OGC standard for displaying georeferences raster data. Here's a tutorial to get started with qgis: http://www.qgistutorials.com/en/docs/working_with_wms.html. And there are numerous discussions about what public WMS resources are available. Here are a few discussions with numerous WMS links*:

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  • Although the SRTM tiles (or CGIAR data - which is SRTM with gaps filled) is probably fine for learning, a more detailed and more accurate DEM is available as OS OpenData Terrain 50. OS also provides a high-resolution DEM called Terrain 5, but that's not available under the OpenData licence.
    – Toby Speight
    Jun 4, 2015 at 7:45
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In the text there is basically an error in that there should be another sub heading between the SRTM data download section and the imagery download section. We provide a methodology for obtaining SRTM data, but you are left to your own initiative to find local imagery data. I will update the text to address this.

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