How to: Open an Orthomosaic
What is an Orthomosaic?
An orthomosaic is an accurate 2D aerial map geometrically corrected such that the scale is uniform and can be used to measure true distance. It is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion and camera tilt.
In general, each pixel is 2.5 cm large due to the use of a standardised GSD of 2.5. Orthomosaic files are saved as ‘.tif’ files, and are available either as a full chunk or as tiles in a grid. Grid size is determined by the data set size.
QGIS can be used to either visualise the full extent file or some of the tiles. Click on this link to download QGIS 3.8 (64bit). Ensure you download the version compatible for your PC. This example highlights the Windows download.
1. Once downloaded open QGIS.
2. To visualise the orthomosaic on QGIS, drag and drop the TIF files into the main QGIS window.
3. Ensure the required coordinate reference system is selected (underlined below). The CRS can be changed by clicking on in the bottom right-hand corner of the screen.
4. The CRS pop up window enables you to select the relevant CRS.
5. Ensure the layers share the same coordinate system as the project coordinate system previously selected. Right click the layer and select ‘properties..’.
6. In the new window select the source tab and the relevant CRS.
7. Drag and drop the DEM into the main QGIS window to visualise it. The DEM represents an estimation of the bare earth elevation.
8. Ensure the DEM layer shares the same coordinate system as the project coordinate system previously selected. Right-click the layer and select ‘properties..’ .
9. In the new window select the source tab and the relevant CRS.
Editing DEM symbology
10. Right-click the DEM layer and select the symbology tab, to alter the colour ramp. The DEM is always set as single band pseudocolour, appearing grey in colour.
i) The colour ramp can be edited, by selecting toggling and selecting an appropriate colour ramp.
ii) The colour ramp can be inverted to ensure the colour and value associations line up. To do this toggle the colour ramp and select ‘invert colour ramp’.
iii) DEM with an inverted colour ramp.
11. Layers can be dragged and dropped within the ‘Layers Window’ on the right-hand side. The uppermost layer is the one that will be visualized in the main window.
12. The 'Identify Features' button (highlighted below) allows you to interact with the map canvas and get information on features in a pop-up window.
ii) To identify the X,Y,Z coordinates of a specific point on the orthomosaic. Select the 'Identify' tool and click on the point of interest. Select ‘Identify All’ in the popup window.
iii) The feature table highlights the X,Y,Z coordinates. Toggle ‘0’ to find the X and Y coordinates, ‘Band 1’ indicates the Z value.
So, you've uploaded your orthomosaic to QGIS. Fantastic! Want to upload a Google Earth layer as a background? Find out how to do so here.