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I'm using ArcMap 10.3 on Windows 10 and I have the 3D analyst, ArcScan, Geo-statistical analyst, network analyst, publisher, schematics, spatial analyst and tracking analyst extensions.

I have a number of .tif files from ASTER and I need to get a Digital Elevation Model (DEM) of Ireland from them. The end goal is to get the altitude and slope for a number of points across Ireland. Here is a screenshot of these images in ArcMap: Arcmap These files are in a different coordinate system to everything else that I have - I have read that it's best to project them to the correct coordinate system after I change them to raster. Is this the correct thing to do? Or should I project them first?

I've used 'Mosaic to new raster' to try and bring these images together into something I can use. I had a value of 1 in the number of bands and selected 16 bit unsigned. This did not work. Here is an image of the result: Results Am I on the right track with mosaic to new raster and just made a mistake, or is this not the way to go about this?

I have tried to project these tifs to the Irish Grid coordinate system (Project Raster), like the rest of my data. However, this doesn't seem to work - what's going on here? Result

in terms of trying to get the slope, I have been using the spatial analyst tool Slope - every time I use it I get this error, why? Error

Summary: What are the steps I need to take to use these .tif files to obtain altitude and slope?

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    You are not wrong with Mosaic to new raster (try a different set of parameters: for example 32/64 bit signed), although it's probably not necessary. Unless you are going to do hydrological analyses or something else that requires a continuous DEM you can use your tif files as they are. You can use "Project raster" to change coordinate system.
    – Martin
    Commented Aug 18, 2016 at 13:55
  • Thanks. Can you explain how I would use the tif files as they are? I have 157 points across Ireland and want to get altitude and slope for each. I did something similar with weather data where I joined the data so each point had a column with rainfall, etc. I assume I can't do that with images (they have no attribute tables) so how do I use the tifs to get the information I need?
    – rakuenvi
    Commented Aug 18, 2016 at 14:52
  • See Luke's answer below, that'll take you through the process swiftly. However, even if tif's usually are storing images, single band (or sometimes multi as well) tif's is one of the most used formats for storing elevation data. Each cell value then corresponds to an elevation, instead of a color code.
    – Martin
    Commented Aug 19, 2016 at 6:54
  • Reviewing this and your previous question, you seem to be thinking about GIS SE as being some sort of online GIS tutor. For your questions to be answered here they should ask only a single question (as per the Tour) that describes precisely what you have tried to answer it yourself, and where you are stuck trying that.
    – PolyGeo ♦
    Commented Aug 19, 2016 at 7:00
  • @PolyGeo I'm sorry. The multiple question issue is symptomatic of my inexperience with GIS - a question that starts as one single question quickly becomes many questions. This one started as "what's wrong with my mosaic to new raster" and now look at it. When an answer is given, I try that solution to only get stuck again and I ask for clarification or add to the question. If I was to ask all these questions separately I would be spamming out of context questions and that's equally as bad.
    – rakuenvi
    Commented Aug 19, 2016 at 7:46

3 Answers 3

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It looks like you have ASTER Global Digital Elevation Model V002 data over Ireland. Therefore you already have altitude - i.e a "Digital Elevation Model (DEM) of Ireland", just in multiple rasters. You don't need to have single output DEM and slope rasters, if you don't you just have to run the below on each DEM tif.

These DEM rasters (assuming they were obtained directly from NASA/USGS) are in a Geographic (WGS84) coordinate system - EPSG:4326.

The "old school" way of getting single DEM + slope rasters is to mosaic the DEMs to a new raster then calculate slope. I'll outline how below. Depending on what you want to do with the data, the newer "Mosaic Dataset" format has much more functionality (like applying a slope function on the fly without the need to store a separate slope raster), but I won't go into that.

  1. Mosaic to new raster. Set your output workspace and raster filename, make sure output pixel type is "16_BIT_SIGNED" and leave everything else.

Mosaic

  1. Calculate Slope. Note: your DEM horizontal units are in degrees so you either need to estimate a Z Factor (DD to Metres conversion), or project your DEM to a coordinate system that uses metres as the horizontal unit before running the Slope tool.

Slope

Output

  1. Add your elevation and slope values to the point attribute table with the Extract Multi Values to Points tool.

enter image description here

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It looks to me from this document that once you complete the mosaic to new raster process, you are already dealing with elevation values (aka altitude?; unless I'm mistaken). It also indicates that you are on the right track with mosaic to new raster as @Martin has commented. You may need to project the data first and get rid of outliers in the data. Edit: By this I mean that you could use the "con" or "setNull" tool to eliminate NoData (or convert to NoData) any non-elevation values from the raster data.

To get the slope, you simply need to run the slope tool which is located in the "Surface" toolbox within "Spatial Analyst Tools". The parameters are very basic. The tool gives you the option for slope of "percent slope" or "degrees slope". EDIT: It looks like your slope is successful (it is just a warning about the units of the z value not being specified).

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  • Thanks, I'm following that document's instructions but I thought that my mosaic to raster failed as everything seems to be at the same elevation (65535) and was also looking to see if there was an easier way, as I was having trouble with this method. I don't know what you mean by get rid of outliers - how can I do that for images without attribute tables? I will try the slope tool, thanks.
    – rakuenvi
    Commented Aug 18, 2016 at 15:13
  • I've just tried projecting these tifs and it doesn't seem to work? Any idea why my projections look they do in my OP question?
    – rakuenvi
    Commented Aug 18, 2016 at 15:45
  • What projection are you projecting from ... e.g. what system was the data in originallly? Does it match other layers that you may have in that grid system? Please edit your question if you can to add that information.
    – jbchurchill
    Commented Aug 19, 2016 at 2:01
  • Re: "EDIT: It looks like your slope is successful". No, the slope output will be garbage as the Z factor was not supplied. See "How Slope works"
    – user2856
    Commented Aug 19, 2016 at 3:16
  • it said the z factor was 1 (the default). Garbage output is still successl in the eyes of the tool which doesn't know any better. Read the output message from the tool. I think i have a pretty good idea how the tool works.
    – jbchurchill
    Commented Aug 20, 2016 at 15:11
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According to Assigning Raster Value to Point Data, use the Extract Values to Points geoprocessing tool to get the slope and the elevation of your points. Use the tool for your slope and DEM raster, and then join the two results by location with your points.

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  • Thank you. I tried Extract multi values to points as there are quite a number of tifs that I need to use. I didn't project the tifs to my coordinate system as that isn't working (see OP question). When I used extract multi values to points all the values that came up in the point's attribute table were 0.
    – rakuenvi
    Commented Aug 18, 2016 at 15:59
  • Try to go to layer properties of one of the tif file, source tab and check the pixel depth and the pixel type. Use these parameters when you use mosaic to new raster tool. Have you tried with 32 bit?
    – CptGasse
    Commented Aug 18, 2016 at 18:41

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