Using the "Change Map" analytic is a convenient way to compare and analyze two different time-series DSMs from successive aerial surveys over the same location, revealing changes and measurements. This is especially true when used to manage quarries, open-pit mines, or for extracted material follow-up.
Shown via a color scale, the red areas represent those that are negative (cut) values while blue represents positive (fill) values.
Shown in another way:
The site needs to contain at least two DSMs of the same location, in two separate surveys in order to compute the change over time from the same geographical location.
Step 1 - Select a survey that contains a DSM layer.
Step 2 - Select the "Analytics" icon from the left menu and select "Change map" analytic.
Step 3 - Select "Launch".
Step 4 - Select the DSMs to be compared.
Step 5 - Click on "LAUNCH CHANGE MAP".
When the analytics is done check the Notification Center. A folder "Changes" is created in the survey data and a layer "change map" is created in the BASE LAYERS tab.
4.1 The "Changes" folder
This folder includes two vector files representing the Cut and Fill areas. These two vectors are for the measure of the differential volume in each area of the site.
Note: There is a tolerance threshold for the creation of these annotations related to a cut or fill. This is 25 cm on the height axis (Z).
4.2 The "Change Map"
This map appears in the "Base Layers" tab.
To display the map, check the box and click the name "Change Map" to open the right panel.
The right panel will display relevant information between the dates selected to issue the Change Map:
- Negative (Cut) values are displayed in red scale
- Positive (Fill) values are displayed in blue scale
4.2.1 Elevation difference range adjustment
It is possible to adjust the range of visible elevation difference computed between the two surveys. It will update the change map displayed and the Cut and Fill volumes indicated. Using either the slider bars on Cut and Fill or a manually input value.
4.2.2 Value at Point
Use the “Value at point” feature to request the raster value (elevation difference) by clicking on any point of the map. Click on a red or blue polygon, it will show the elevation difference compared to the survey selected in the change map.
Use a point annotation to extract all raster values under the point and compare the DSM values.
Open the point annotation from the measure panel > Drop a point where needed > a pop-up window will appear with the relevant information. If necessary, save the point to keep that information.
Three files are generated as deliverables and can be exported if necessary from the Downloads section:
- A vector "geoJSON" file containing the polygon vectorized from the raster result in terms of Fill (positive values)
- A vector "geoJSON" file containing the polygon vectorized from the raster result in terms of Cut (negative values)
- A raster "tif" file representing the height difference between both DSMs compared