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Crop Agriculture

Course 5A - Downloading & Using Free High Resolution Elevation Data in Free QGIS Software

(See Step-By-Step Guide Below)

Depending on your location in the USA, you may have FREE access to 1-meter bare earth raster elevation data (elevation value every 1 x 1 meter pixel) or 1/9 arc-second bare earth raster elevation data (not square pixels; average resolution in USA is about 3-meters) from The National Map (USGS; availability is shown below). Both types of elevation layers should be LiDAR-based (you can check the metadata to be certain, though). If you are using a different elevation source than The National Map, or data from another country, the QGIS steps below may still apply.

You can view the metadata to find the year or dates the elevation values represent (metadata link is shown in steps below). The vertical and horizontal accuracy of LiDAR-based elevation data varies based on many factors (such as the density of of LiDAR points that the raster is developed from), but based on GIS Ag Maps' experience the data is very much "usefully accurate". At the end of the steps below, there is a comparison between elevation and soil darkness that conveys accuracy. You can research accuracy levels if you like, of course, but we suggest using the data to make a determination about whether is useful for a particular purpose. ELEVATION RASTERS CAN BE USED TO DERIVE OTHER LAYERS, SUCH AS DRAINAGE AND TOPOGRAHY - THE GUIDE BELOW SHOWS HOW TO APPLY FOR FREE DATA FOR VERY USEFUL AND SIMPLE PURPOSES, SO YOU CAN EASILY USE SOME OF THE USEFUL FREE DATA AVAILABLE.

If you need free QGIS software you can access Course 1A (describes how to download free GIS software), download software directly from this website, or go to the QGIS website (https://qgis.org/en/site/forusers/download.html; we recommend using a 2.18 version because the QGIS User Guide and Training Manual are designed for 2.18, though neither the User Guide nor Training Manual is necessary here).

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Steps to Start Using High Resolution Elevation Data for Free

1) Click here to go to (USGS) National Map download page or paste the following address in your browser:

https://viewer.nationalmap.gov/basic/

The following USGS TNM (The National Map) page will open:

 

2) Open Elevation Products, then click Show Availability next to 1 meter DEM (Digital Elevation Model) to show locations with 1-meter data.

 

3) Click Show Availability next to 1/9 arc-second DEM to show locations with 1/9-meter data.

 

4) For this example, 1/9 arc-second data will be used. Click the Layers icon in upper right to to help find an area of interest. In this case, USGS Imagery and Topo is used; it shows background imagery and topographic information.

 

5) Zoom to an area of interest by scrolling in or using the +- tool. A crop field is applied for this example, though the elevation data can be applied to a multitude of other surfaces.

 

6) There are different ways to establish the are for data; for this example, the Box/Point tool will be used. Click the radio button next to Box/Point and draw a box around area of interest by left-clicking cursor and dragging a box around an area (it is important to note that data will be part of a larger area [tile of data]; also, if the selected area happens to encompass more than one tile, all tiles will be available for download). After selecting area, CLICK FIND PRODUCTS.

 

7) Available product/s will be shown and will be available for download (in this case, elevation products).

 

8) Click download to have download window appear (you can also click other links to view background information on data). We recommend that you set your internet browser to always ask where to save data.

 

9) Locate the downloaded compressed folder on your computer and unzip (decompress) it. If you need decompression software, we recommend free 7-Zip (outside page; opens in new tab). If using 7-Zip to decompress, you can right-click on folder, then navigation to7-Zip, then click Extract Here.

10) As is stated near the top, if you need free QGIS software you can download it from this website or go to the QGIS website (https://qgis.org/en/site/forusers/download.html; we recommend using a 2.18 version because the on-line User Guide are designed for 2.18, though a Guide is not necessary here).

Open Free QGIS software by double-clicking on the QGIS 2.18 folder that should be on your desktop to open the folder, then click on QGIS Desktop (for whichever version you have) to open the software (may take a few minutes to open, depending on speed of computer). Then navigate to Layer > Add Layer > Add Raster Layer to add the elevation data layer, as is shown below.

 

11) If using 1/9 arc-second data, add the Disc Image File (shown below). If using one-meter data, also add add the Disc Image file (though the included files in a one-meter download will vary from the graphic below).

 

12) The elevation layer will be added in grayscale (lighter shades are higher elevation). Elevation values will be in meters above sea level - and elevation layer in feet can easily be produced and will be produced for this example (as is shown in later step). At this point you can zoom into a pixel and measure its dimensions.

 

13) Add high-resolution imagery by navigating as follows: Web > OpenLayers plugin > Bing Maps > Bing Aerial (or Bing Aerial with labels). Decent internet speed is necessary for this; if you have slow internet speed, see Course 1C, which shows how to download high-resolution imagery.

 

14) Imagery will appear over the elevation layer in the Layers Panel (to move layers in the Panel, drag and drop them; though there is no need for this here). If you zoom into a pixels after the imagery has loaded (uncheck Bing Aerial view the elevation layer), you will notice that the dimensions of the elevation pixels has changed from when you first loaded the elevation data (if you measured the dimensions in Step 12). QGIS has done this automatically so that the layers line up (as they have different spatial references).

 

15) Zoom in to an area of interest with the Magnifying Glass tool (has plus sign) by either clicking on the screen or dragging a box. You can zoom in more than this (as is shown in next step); however, if you zoom in too far, the imagery may disappear.

 

16) It is important to note that imagery can change, depending on amount of zooming (scale).

 

17) If you would like the elevation values in feet, access the Raster Calculator as shown below (click Raster Calculator).

 

18) Calculate a new layer, by multiplying the meters elevation layer by 3.28084, as shown below (making sure to locate and name the output layer).

 

19) A new Elevation Feet layer will be produced and will load into the Layers Panel, as shown below.

 

20) If you click the Identify Tool (lower case "i"), then click the imagery when the Elevation Feet layer is shaded (make sure the elevation layer is shaded by clicking it), elevation values will appear on the screen. Click on the area of interest to estimate the high and low elevation values; these values will be used in following steps.  

 

21) Turn off Bing Aerial by clicking the check mark in the Layers Panel; the elevation layer can now be viewed. When viewing the elevation (particularly at a field-scale), there will not be much symbology difference (it will probably appear as virtually the same shade of gray) because the overall elevation layer is much larger is size and represents a larger range of values, and is scaled to that larger range of values. A closer view, such as the one in this example, can be improved by using the high and low elevation estimates from the previous step.

 

22) Change the symbology range (and, therefore, grayscale range) by right-clicking on the elevation layer, then clicking Properties.

 

23) When the Layer Properties window appears, modify the range of elevation values by entering as Min and Max near the bottom of the window, then click OK. In this example, the values of 779.0 and 788.5 are used.

 

 

23) The elevation layer will now be symbolized base on the range of custom values that were entered (the lowest elevation has the darkest shade and the highest values has the lightest shade; black to white, with shades of gray between). You may need to modify the Min and Max values a number of times to be satisfied. In this example it is easy to see, by comparing the elevation to the imagery below it, that lower elevations correlate to darker soil (though there seems to be an overall tilt of the landscape to lower elevations toward the north; north is up).