Digital Elevation Models
ArcMap and ArcScene with 3D Analyst extension allow you to display and analyse digital elevation models (DEMs). With this extension you are able to perform the following tasks:
- Determine height at any location.
- Create realistic surface models (Elevation Tints, Shaded Relief, and vector/ortho photo overlay)
- Create 3D perspective views
2. Contour Lines
In the first labs, you worked with NTDB 1:50,000 data; in this lab, you will use ‘high resolution’ local municipal data from the City of Prince George for the area around UNBC. The first file to load is the contours for the UNBC area.
- Login to lab machine and create a new folder dem under your geog205 folder.
- Start ArcMap with new map file and add contours_1m from L:\labs\geog205\unbc
- Click the line symbol below contours_1m layer and change the line colour to brown. Click OK.
- Use the zoom, and pan buttons to zoom and pan your map.
- Click to reset your map to its full extent.
- At this point you may want to resize your ArcMap window to make it bigger.
A closer look at the data:
- Click on the Identify button and place the cursor over any line and click to display the feature’s attribute information.
- At the bottom of the table it will indicate what height the line represents (ZVALUE in metres).
Notice in the lower right corner, UTM coordinates are given.
Q. What is the contour interval? (query two adjacent contour lines)
- Add lakes, trails, creeks, roads and lookouts from L:\labs\geog205\unbc
- Change the layers’ colours to follow logic and cartographic conventions (blue for water etc.)
Q. Determine the elevation at the two lookouts, by querying the contour line closest to each one. What are they?
Later in the lab, we will see how GIS can be used to create slope values across a surface, but here you will manually calculate slope using the difference in elevations between two points and the distance between them:
Slope (percent) = 100 * vertical rise / horizontal run
Slope (degree) = Inv tan (vertical rise / horizontal run)
The measurement of vertical rise and horizontal run must be in the same units. On a hardcopy map, we would calculate vertical rise from the difference in contour heights, and the horizontal run, given by measuring the distance with a ruler (and converting to actual ground distances).
There is only one small stream running into Shane Lake (south-west area of the lake near 510676, 5971143). Query the highest and lowest contour lines it crosses, this should be = 826m / 814m – and assume the lake surface is 1 metre lower at 813m. The distance using the measuring tool comes to about: 129m. The heights and the distance are both in metres.
Vertical Rise = 13m
Horizontal Run = 129m
Slope (percent %) = 13 / 129 * 100= ~10% (10.078%)
To calculate slope in degrees: Calculate using the inverse tangent of height change / distance. Use the computer calculator: Start->All Programs -> Accessories -> calculator, Click View -> Scientific.
In this example: slope (degree) = Inv Tan (13 / 129) or Inv Tan (0.1) = ~ 5.7°
Finding the slope gradient of the two trail sections heading down (east) from the water tower to UNBC
- Use the Identify tool to find them – ensure you are querying the trails layer.
The first starts near 511250, 59 71700 and ends near 511545 5971700; the second ends at 511956, 5971700
Q. what are the slope gradients in percent – which of the two trail portions is the steeper?
Q. How could you visually tell which is steeper without making any calculations? (by reading the contours)
3. Digital Elevation Models (Grids)
In order to display and perform 3D analysis, we need to enable 3D Analyst extension
- Click Customize-> Extension and choose 3D Analyst in ArcMap. This will enable the 3D Analyst extension
A DEM (grid) named elev raster dataset has been created for you to save disk space and not have everyone create it themselves from contours)
- Add elev from L:\labs\geog205\unbc to ArcMap and turn off the contours_1m
- Again click on i button and look at the attribute data and set elev as the query layer in the popped up window.
Go to the lookout points and notice that the values are almost the same as the contours lines (which are integer data). The grid values have been interpolated to create a continuous surface, creating decimal places.
The Grid DEM appears as gray, we can change to colours (hypsometric tints) if we wish:
- Right click on elev and Zoom to layer and double click on the colour bar below elev.
- In the colours ramps box change the current colours scheme to ‘a suitable one’. Click OK. Note that you can ‘invert’ a scheme if needed (= reverse the colours)
It is a good idea to reclassify elevation data into groups for hypsometric tints. We are going to reclassify the display on elev into six classes with an interval of 50.
- Right-click on elev and select Properties, [or Double click on elev to open the Properties window.
- Click Symbology tab. Currently the dataset is displayed in stretched mode. Click Classified in the Show box at left
- Change the class number at right to 6 and click Classify button beside. Here you can change the range value for each class.
- In the Classification window, look at Break Values box at lower right.
- Change the break values to 650, 700, 750, 800, 850 and the highest respectively.
- You may need to change the colour ramp back to your colour selection
- Click OK. Click OK again.
Now you should have 6 classes starting from 650 and 50 as interval. Does your map look different?
To change individual colours, you would double-click on each box in the classified panel (under the symbol column).
3D Analyst extension allows users to generate secondary products from DEM. e.g., hillshade, slope, aspect etc.
Working with 3D Analyst – ArcMap Toolbox
- Click ArcToolbox icon .
- Once the toolbox comes up click the plus signs to navigate down the 3d Analyst tools (should be the first toolbox) to expose the Raster Surface set of functions (tools). The next set of exercises should be in this group of functions.
3D Analyst extension allows users to generate secondary products from DEM. e.g., hillshade, slope, aspect etc.
- In ArcToolbox, click 3D Analyst tools -> Surface Raster-> Hillshade
- Fill in the boxes using:
- Use the elev layer as your input
- Save your output file as hillshade in your dem directory
- Leave the other values as they are (the default values are the one we want
- Take default value for the rest and click OK.
- The newly generated hillshade (hillshade) will be added to ArcMap automatically.
You can use the query button i to query pixel value. [Ensure hillshade is the layer being queried]. What do they mean – query a dark area and a light area. They are simply digital numbers ranging 0-255. Low=black (dark SE facing slopes) and high = white (NW facing slopes).
Combining shading and tints (and contours):
By placing an elevation grid on top of a created hillshaded grid and making the elevation grid transparent, you obtain a realistic image of the landscape. You can add other layers such as roads or streams to further increase the informational content in the display
- Make sure the elev is placed above the hillshade. Rearrange the order of each layer if necessary.
- Right click elev and choose Properties.
- Click the Display tab. Type a value in the Transparent box to set the percent transparency. A reasonable value is 50 percent. Click OK. You should now see the hillshade raster through the elevation.
- Right-click elev and zoom to layer to see the whole map area
- Try 70 or 80 % transparency
Create slope from DEM (elev)
Following the same procedures for making the hillshade – create a slope raster layer (in degrees) in your dem directory from the elevation dataset
3D Analyst tools -> Surface Raster-> slope
- Query the slope values along the trail line which you calculated manually in section 2. They should be approximately the same – they may vary slightly due to the process of interpolation.
Examine Lidar DEM
We have examined elevation data at 5 metres resolution (elev). Now we are going to explore the Lidar data. LIDAR (Light Detection And Ranging) is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, using pulses from a laser. This generates a high resolution DEM.
- Turn off all layers and,
- Add lidar_unbc from L:\labs\geog205\unbc
Now create a hillshade (lidar_hsd) and slope (lidar_slp) raster from lidar_unbc and visually compare the result to the one you created with elev
Calculate the approximate height of the glacial lake beach lines, east of campus by viewing the elevation below and above a beach line. – View the hillshade or slope but query the DEM (elevations).
Q: What is the approximate height of the glacial lake beaches?
4. Using DEMs with ArcScene
ArcScene allows you the ability to create a 3-D view terrain mode. We will use the raster grid DEM surface for height.
ArcScene lets you:
- Build many-layered scenes
- Control how each layer in a scene is symbolized and rendered
- Control how each layer in a scene is positioned in 3D space
- Select features in a scene using their attributes or their position relative to other features
- Navigate around a scene
- Specify the coordinates of the observer and target for a viewer
- Click Start -> All Programs -> ArcGIS -> ArcScene. An ArcScene window looks similar to an ArcMap window. The left part is the table of content and the right part is the display area.
- Add elev from L:\labs\geog205\unbc to ArcScene
- Remember this is data you are adding to this new program – but it works the same as ArcMap
- Right-click -> properties -> symbology to select your favourite colour scheme as above
Setting Scene Properties
- Double click elev to open the properties window.
- Click Base Heights tab and click the button beside ‘Floating on a custom surface’ – this should have the elevation layer as the surface’. ‘OK‘ to close properties box
- Right click Scene Layers in the table of contents and choose Scene Properties
- Select 2 for Vertical Exaggeration. That will result in more contrast in elevation. Click OK
- Click 3D navigation button and place your mouse cursor somewhere in the display window.
- Hold and drag mouse button to view the surface from different location.
- Scrolling the middle mouse button allows you to zoom in and out.
Using contours in a scene
Contours are a familiar surface representation and can enhance a terrain model. You can display contour features in a 3D scene by setting the base height of the contours from their value in the feature table. The example here shows contours superimposed on a terrain model.
- Add contours_5m from L:\labs\geog205\unbc to the scene
- Right click contours and choose Properties
- Click Base Heights tab and check ‘Use constant value or expression to set height for the Layer’.
- Click the Expression Builder button (to the right). Here you build up expression to be used as base height.
- Leave this value as 0. This will result in the Elevation field used as base height.
- Click OK to apply the changes. The contour lines should be displayed in the scene.
Draping 2D data on the surface
You can drape 2D data on the surface to make 3D effect.
- Turn off contours_5m layer.
- Add data creeks from L:\labs\geog205\unbc to ArcScene. You can see that the creeks layer does not show on the top of surface. It is underneath the surface. This is because creeks is a 2D layer and you need to set the base height to make it show on the surface top.
- Now right click creeks and choose Properties.
- Click Base Heights tab and click the button beside ‘Floating on a custom surface – this should have the elevation layer as the surface’
- Click OK to apply the changes. The creeks will be displayed on the surface.
Similarly you can drape other vector layers to the surface.
- Repeat these steps to drape roads, trails and lakes from L:\labs\geog205\unbc to the surface.
5. Drape an image and fly through
Overlay (drape) an aerial photo over a DEM
You can also drape an image on top of the terrain model, such as aerial photos, satellite imagery, hillshaded images.
- Turn the check mark for elev OFF
- Add dataset unbc2002.tif from L:\labs\geog205\unbc to ArcScene. (it may ask you to build pyramids – click YES)
- The image will be loaded into the screen but does not have any height values. To assign height values, right click on unbc2002.tif and set the properties in the same manner as you did for the creeks and other layers
You can change exaggeration of the terrain by changing the 3D scene properties.
- Right click on the default Scene Layers and choose Scene Properties.
- Adjust the vertical exaggeration factor to 5. To finish click (apply followed by) OK. Better or worse? You can also type in an intermediate value, such as 3
You can use the left mouse button to change the view location and the middle wheel button for zoom in and out.
You can navigate through a scene using the Fly tool (the bird button) – this bit may be challenging..
- Click the Fly bird button. The cursor changes to indicate fly mode is active.
- Click once in the center of the scene. The tool enters the suspended state.
- Click the mouse to move forward. Right-click to move in reverse. Successive clicks in either direction increase the speed. Speed is indicated in the status window (if you can see it)
- Click the opposite button to slow down incrementally and then stop. Press Esc to stop movement.
6. Online mapping websites – DEM and relief features
- Zoom to UNBC area
- Click ‘I want to’ and choose Change the visible map layers
- Expand Imagery and turn on Ortho 2006 (near bottom). You can view changes by also turning on 2003 and flipping 2006 on and off
- Add contours 2003 and 1993 (pre-UNBC)
- Note that not all contours appear before you zoom in a lot
- What is the contour interval when you are closely zoomed in?
- What is the elevation of the agora (centre circle)
B. IMAP BC – Geographic Data Service: https://maps.gov.bc.ca/ess/hm/imap4m/
- From the imap BC, click ‘I want to’ -> Add Provincial Layers-> Imagery
- Add Data BC Layers hillshade black and white (then click OK)
- Turn this layer off and add hillshade colour (refresh). This includes shading and tints
- Note the two options: SW shading (simulates ‘afternoon’ lighting and NW (evening lighting)
- ‘painted relief’ option (at bottom) gives a higher resolution tinted relief – but only available for SW BC
- Contours can be added from the ‘Base Maps’ folder – 1:250,000 contours will appear first, and 1:20,000 (1o metre interval), when you zoom in
7. 3D scene using a larger extent (Smithers area)
Elevation and other data for Smithers are located in in L:\labs\geog205\smithers. There is a collection of data at different geographical extents for students to play around with. In this section you are to duplicate the work you did above, but using data from around Smithers.
Description of data:
In the landsat_8_2015 folder:
large_extent_smithers_area.tiff — This is a Landsat 8 Satellite Image of an area containing Smithers and a large area south.
large_extent_smithers_area_dem.tiff — This is a DEM of the same area.
smithers_area.tiff — This is a smaller subset of the same satellite data, but only containing the area around smithers
smithers_area_dem.tiff — This is the DEM of the same smaller area.
In the main folder:
Assorted vector layers (shape files) for the area (smaller than the smithers_area raster data) including: contours, lakes, rivers (as well as many other local features).
Things to try:
Large extent 2D combination
- In a new project, load the large extent layers into ArcMap (the satellite image and the DEM)
- Create a hillshade layer from the DEM (perhaps call it smithers_hillshade_large.tif)
- Stack the layers with the DEM at the bottom, the hillshade next and the satellite image on the top
- Choose a tint colour scheme for the DEM
- Play with the transparency, brightness and contrast values of each layer to create an area that has the satellite data with shading and tinting effects (using the Display tab in the layers property panel)
- Try this combination with some positive results:
|DEM||10 contrast and -20 brightness|
|Hillshade||10 contrast, -10 brightness, and 25 transparency|
|Satellite||20 contrast, -10 brightness and 34 transparency|
Navigate around the scene you created (zoom and pan) to see the effects you have made. Save your work (perhaps as smithers_large_2d)
3D presentation with the smaller extents
- Open a new project in ArcScene and load the smithers_area_dem.tiff layers into the project
- Create a hillshade from the DEM (smithers_hillshade_small.tif for instance)
- Open up ArcScene and load the smaller DEM, the newly created hillshade and the smithers_area.tiff file (the smaller satellimage)
- Create a 3D scene as you did earlier in the lab with these three layers
- Experiment with transparencies, brightness and contrast (as above) with the layers (try using without visualizing the DEM as well) to determine whether the 2D shaded effect is beneficial in the 3D environment
- Load in some of the vector layers to see how they add to the scene.
Save your work to a new map file and arcscene file.
8. Practise your mapping skills
Load all the vector data in the Smithers folder into the ArcMap project you just worked on (the 2D scene). Using this Smithers data set, review and practice your skills from lab 1 through 4 (read through the labs for ideas).
Can you find features in the 2015 image that are not symbolized in the vector data set? Hint: go to 6123332, 6070810. What is this feature?
Approximately 5 kilometres north of this feature is the switch back roads to a mine. Is this mine symbolized in the vector data set?
Where are the glaciers? Right-click the layer glacier, click Zoom to Layer. Symbolize glacier in red and toggle it on and off.
Repeat any of the procedures from labs 2-4 for practice with this more exciting mountain dataset
Once you are done the work
- Quit from ArcGIS
- Logout from Osmotar and local machine