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Finding Stockpiles with QGIS

Finding Stockpiles with Quantum GIS Christmas Island has a number of stockpiles of low grade phosphate that were set aside during the mid 1900s. The following exercise will demonstrate how to identify and measure these stockpiles using Laser Digital Elevation Models (DEM) and ortho-photography. To begin, open QGIS in Internet Explorer.

Opening the Laser DEM Project

1. Select View spatial data from the left hand menu. The View Data – Explore Mapping Projects page will appear.

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2. Click the Laser DEM link from the Project Name list. If a file download box appears, click

‘Open’. The QGIS LaserDEM project will open. This project has been set up to automatically load the spatial layers relevant to this task including the Laser DEM layers.


Selecting Layers, Zooming and Navigating

1. All spatial data layers in the project are listed in the sidebar to the left of the image window. Switch layers on and off by clicking in the box to the left of each layer title. When the box has a cross in it the layer will be displayed in the image window.

2. Use the tools highlighted below in the top menu bar to zoom in, out and pan. Clicking Full Zoom will display the full extent of the selected layers.

Identifying Stockpiles Stockpiles of phosphate are identifiable using the Laser DEM imagery as it shows the ground height elevation. Unlike satellite images, Laser DEM capture excludes objects like tree tops and buildings, allowing accurate ground elevation measurements. The Laser DEM data is available for the eastern section of the island and is displayed in 2km x 2km tiles. To view the Laser DEM tiles:

1. Click the zoom full button from the top menu bar

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2. Turn off the satellite image (Christmas Island 2006 Satellite Image) and the aerial photography (Orthophotography 1987) in the left sidebar.

3. By zooming in to examine relevant areas, stockpiles can be identified from the LaserDEM

layer. Some stockpiles have a circular appearance because they were made with dump trucks; others are tabular as the soil was shifted using scrapers.

The image on the left is an example of a circular phosphate stockpile, on the right is an example of a tabular phosphate stockpile.

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Note: Occasionally the image may incorrectly show a building as a tabular deposit. To ensure a stockpile has been correctly chosen locations should be crosschecked with the corresponding satellite (2006) and ortho-photograph (1987) imagery.

4. Once a stockpile has been located its location can be crosschecked with the corresponding satellite image. To do this the LaserDEM layer will need to be turned off. As each tile of the LaserDEM is listed as a separate layer in the side bar the first step is to determine which tile you are working on.

a. Ensure the grat_2k layer in the side box is turned on and highlighted. b. Click the Identify Features icon from the top menu, and then click on the map within

the tile you are working on. The Identify Results window will display. c. The number next to the ‘DEMID’ is the tile number you are working in.

5. In the left sidebar, turn off the LaserDEM layer that you are working in. In this example that would be layer L2059_SD3.

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6. Turn the Christmas Island 2006 Satellite Image on.

Note: To view the Orthophotography layer you must switch off the satellite layer, and turn on the Orthophotography 1987 layer.

Estimating Stockpile Height and Volume

To estimate the height of the stockpile you will need to extract the point height file which matches the tile you are currently working on. 1. Navigate to \\cigis\LaserDEM\als_height, generally located on your C drive; this is where the

point heights are stored as shapefiles. 2. Double click to open the als_height.zip

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3. Locate the three height files corresponding with your tile DEMID number. In our example these are l2509_gnd.shx, l2059_gnd.shp and l2059_gnd.dbf.

4. Hold Ctrl and select the files required. Click extract. The Extract window will display. Note: If you are not given the option to extract the files they can be copied and pasted to your required directory.

5. Use the Extract to drop-down list to specify where you would like to save the files.

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6. Return to your open QGIS project and turn the Laser DEM layer back on. It is important to zoom in closely on the deposit you wish to investigate.

7. Click on the Add Vector Layer icon to add the point height layer. The Add Vector layer window will display.

8. Click Browse to navigate to the height files (.shp, .dfb and .shx) you previously saved. Locate the .shp file and click Open.

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9. Check that the location has appeared in the dialogue box in the dataset row. 10. Click Open. The height layer will be added to the map as a collection of points.

11. To measure the hight of a point on the map, click the Identify Features icon then click on the point. The Identify Results window will display. The height value is listed.


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12. To find the deposit’s height click a point in the centre of the deposit, then a point at the base.

Remember: you can turn the height point layer off and on by clicking in the sidebar. Height at centre of the deposit Height at base of the deposit = 200.83m = 191.37m

13. To calculate the height of the deposit, subtract the height at the base from the highest point. For example: Height = 200.83m - 191.37m = 9.46m

14. To measure the width of the deposit, first turn off the height layer, and then click the Measure Line icon.

15. Click at one edge of the deposit, then at the opposite edge. Right click to end the measurement line. A dialogue box will appear calculating the length of the line drawn.


Width = 154.30 m

16. Input the height and width of the stockpile into the relevant formula to discover its volume. Formula for a cylindrical deposit:

Formula for a tabular deposit:

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