Strayos provides multiple options for calculating the volumes of stockpiles. Users can select among a combination of different surface layers and base planes to accurately calculate the volume in different use cases.
Strayos provides three different volume calculation surfaces - 3D Model, Digital Surface Model (DSM) and Digital Terrain Model (DTM).
The 3D Model is a high quality reconstruction of the surveyed area generated by stitching together the images captured by the drone. This model can be used for both visualization of the surveyed site as well as for volume measurements.
Digital Surface Model:
The Digital Surface Model (DSM) is a computer generated representation of the terrain’s surface. DSM takes into consideration the objects on the terrain like cars, conveyors, buildings, trees,vegetation and other equipment. DSM provides accurate elevation values with more resolution and therefore provides accurate volume measurements.
Digital Terrain Model:
The Digital Terrain Model (DTM) is also a computer generated representation of the terrain’s surface but unlike the DSM, the DTM excludes any objects on the surface like buildings,trees,vegetation and equipment. The DTM contains only the terrain elevation values. Due to this nature, the DTM is very useful for measuring stockpile volumes accurately when there are conveyor belts overhead.
Comparison of Digital Surface Model (left) and Digital Terrain Model (right).The Digital Terrain Model excludes the conveyor belts.
Strayos platform provides five different base plane for volume calculation for stockpile reporting.
- Lowest Fit means a horizontal base plane at the lowest elevation of the boundary points. Lowest Fit is the most common used type for stockpiles which are heaped against a bench or other objects. It will set the bottom horizontal plane at the elevation of the lowest point has been clicked.
- Linear Fit is the best method for stockpiles which have clear boundaries and located on a flat ground. It will generate a best fit plane from all clicked points and calculate the volume based on best fit plane. Note: You can always select points on the surrounding ground surface. Including extra ground area will not bring any significant difference for this method. If you are not certain about the stock pile boundary, just include more ground area. On the other hand, if polygon points are clicked on the top of a stock pile, it could bring a larger difference.
- Average Fit works for stockpiles which sits on a flat ground and have a clear visible boundary. It sets the bottom plane as a horizontal plane with an average elevation of all the clicked points.
- Manual allows you to enter custom elevations, Grade and Azimuth values by rotating the base plane either by directly input Elevation, Grade and Azimuth values or by dragging in the 3D model. Manual options can be used for stockpiles that are in slope OR on a inclined plane.
- Triangulated Fit : It works for large and long stockpiles. It will run triangulation algorithm on the polygon, and calculate volume based on triangulated base planes. The base plane is not a single plane, but a set of triangles.
STEPS TO PERFORM VOLUME CALCULATIONS
- Draw Polygon
When the user draws a polygon around a stock pile, the app records the points clicked by user to draw the polygon and the elevations at those points.
The app then filters the points on the 3D model that are inside the polygon from other points on the model.
The filtered points are used to create a mesh of data points. The mesh is then used as top mesh of the pile.
Then Strayos project the top mesh to a bottom horizontal plane, and enclose the space with vertical faces.
- Select Volume Types
Strayos provides three types of volume metrics.
Lowest Fit is the most common used type for stock piles which are heaped against a bench or other objects. It will set the bottom horizontal plane at the elevation of the lowest point has been clicked.
Manual Fit is to manually input the elevation of bottom plane and users are also able to select plane on the model. You can select custom grade/inclination for piles that are in slope.
Linear Fit works for stock piles which sits on a flat ground and have a clean surroundings. It sets the bottom plane as a plane with an average elevation of all the clicked points.
- Cut and Fill
Cut Volume refers the volume you would have to remove from the area (stock pile) in order to flatten the ground. Cut Volume is always a positive value.
Fill Volume refers the volume you would have to fill the area (for example, a hole) in order to flat then the ground. Fill Volume is always a negative value.
Total Volume = Cut Volume + Fill Volume
The blue part represents the fill area. The red part is cut area.
- Stair Volume
To measure the stair volume of a bench, users only need to draw a polygon around the bench and select ‘lowest fit’ to do calculation.
- How does Strayos Volume Calculation Algorithm works
The core idea is to arrange the filtered data points in a specific order and divides the points into many small object. For each small object, we calculate the signed volume (opposite cross product orders).
For example, red object and blue object are the small object we get. Give them three vertices v1,v2 and v3. To calculate the volume of red object, we arrange them in an order (v1 → v2 → v3), and get a negative value. To calculate the blue volume we arrange them in an opposite order (v3 → v2 → v1), so that we get a larger positive value. Add them together we get the pink volume. Add all the small pink objects together we get the total volume.
DSM and DTM based calculations:
DSM and DTM are georeferenced raster images which provide the following information:
- Elevation value at each pixel of the raster
- Area of each pixel
Based on the above 2 values, we filter the area of interest from the whole raster and calculate the volume. Depending on the altitude of the drone flight, it is possible to achieve high resolution DSM/DTM and therefore calculate the volume more accurately.
TIPS TO IMPROVE VOLUMETRIC ACCURACY
- Clear stockpile boundaries : It’s always good to have a clear visible boundary of the stockpile. If the stockpile is against some other entities, and you cannot select boundary points on a flat ground, it’s better to use lowest elevation base plane volume.
- Tight Bounds : Click as close to the entity boundary as possible while adding points to the polygon being drawn.
Add as many points to the polygon as possible since this helps improve the accuracy of volume calculation.
If the base elevation of the stockpile is known the user should utilize Strayos' reference elevation options to obtain the best results.