This article is a workflow on how to design a Water Retention Pond in the Virtual Surveyor app. You can design a water retention pond on the terrain, create the grading for it, estimate cut/fill volumes, figure out balancing of the materials, export the surface as a machine control model, and track the build progress over time.

Overview

• Create a Baseline
• Create a New Timestep
• Finding a Good Location to Build
• Draw the Design Line
• Design the Retention Pond
• Create a Grading from the Surface Design
• Material Estimation
• Material Balancing
• Export a Machine Model
• A Deeper Look: Detailed Analysis
• As Built
• Extra: Calculate Water Volume 
• Credit

Create a Baseline

We'll want to create a baseline Timestep as a starting point and foundation of the project. This Timestep includes the elevation model (the DSM) and the orthophoto from the initial survey.

To setup a baseline, you'll want to show the Timeline using the button from the View tab or status bar and change the name of the first Timestep to 'Baseline'. 

Create a New Timestep

Next, we'll need a new Timestep that includes a copy of the initial DSM and orthophoto that we can modify or alter as we design the retention pond:

1. Click on Add Timestep.
2. Type in the design name for the pond: Retention Pond Design.
3. Copy the DSM and Orthophoto from the Baseline.
4. Switch to the Retention Pond Design Timestep.
5. Paste the DSM and Orthophoto into the new Timestep.

Finding a Good Location to Build

The retention pond should be built to capture the flow of water across the terrain. We can turn on the Slope Direction and Contours lens' to help us see the flow of water. Then, we can identify an area on the terrain where the flow of water converges (yellow ellipsoid). This is where we'll construct our pond.

Draw the Design Line

We've zoomed in to the region we want to create a water retention pond on, and we'll use the contour line to help us draw the primary design line for the base of the water retention pond. We'll draw the design line while in the 2D view mode:

1. Draw the design line using the Arc polyline with the contour line as a guide. 
2. Right-click and select Edit Vertex from the mini toolbar.
3. Change the Edit Vertex Mode to Z and click Select All Vertices. 
4. Set the elevation of the design line to 2870 m in the Z Vertex Coordinator box.

Design the Retention Pond

 We need a few elements to design the water retention pond, including a shoulder, the floor and some proper slopes to safely hold the water in. Then we can create a mockup of the design to ensure our design model is on the right track.

Create a shoulder on the design. We want to create a wide shoulder that we can walk along to inspect the retention pond when necessary.

1. Right click and choose Offset Line from the minitoolbar.
2. Set the Z Offset Method of the design line to create a shoulder at ΔZ 0, with a Distance Drawing Guide of 1 m.
3. Left click on the outer guideline to create the offset line for the shoulder.
4. Offset the design line again to create the inner portion of the pond by setting the Z Offset Method with a Slope of -30 degrees and a guide with the distance of 1.5 m.
   1. Left click on the inner guideline to create the offset line for the floor of the pond.

Model the floor of the retention pond. Draw another line using the Arc tool using the inner lowest edges of the curve for the bottom of the pond. 

1. Arc the line to follow the contour line as closely as possible (best in 2D view mode).
   
2. Use Edit Vertex to Interpolate the elevation of the line using Interpolate Z in Linear mode (enable Transparent lens to see better).
   
   1. This will drop the arc below the current ground elevation and linearly interpolated from left to right. 
3. Create a surface using the four drawn lines. This will be the baseline design surface of the water retention pond.

Create a mockup model of the pond. To quickly verify that the design is going to work, highlight all the lines and modify the terrain, which should give us a good look at what the base of the retention pond will look like.

Create a Grading from the Surface Design

We'll connect the retention pond design with the surrounding terrain by creating a grading of the surface design:

1. Select the surface design.
2. Navigate to the tools tab.
3. Set the Cut Slope to 30 degrees.
4. Set the Fill Slope to -30 degrees.
5. Click the Grade button.

We'll inspect the grading from multiple angles to make sure all the elements we need of the retention pond are included before getting a material estimation.

Material Estimation

We'll use a boundary around the retention pond surface design to create the cut and fill for material estimation.

1. Draw the boundary around the retention pond.
2. With the boundary selected, navigate to the Tools tab and set the Use dropdown option to Timestep.
3. Select the Baseline from the Reference State dropdown menu.
4. Click the Cut/Fill button.
5. Delete the boundary as we won't be needing it anymore.

Material Balancing

Optimally, we want to use the dirt from the cut to create the fill without having too much excess material. For how the current retention pond design is placed on the terrain, we have much more cut material than fill with the Net Cut at 34.49 m³.  We can correct this by increasing the elevation of the design's functional surface so that we have less cut and more fill. To correct the cut/fill volume:

1. Delete the graded surface and the modified terrain since we won't be needing them anymore.
   
2. Select the original design surface and navigate to the Tools tab.
3. Set the Offset Z to .10 m and click the Offset Z button.
   1. This moves the entire pond design up by 10 cm, creating less cut and more fill.
4. Regrade the surface design by clicking on the Grade button.
   1. Ensure the Cut and Fill Slopes are still set to 30 and -30 degrees respectively.
5. Recalculate the Cut/Fill.

After recalculating the cut/fill portion of the model, we can see that there is still more cut than fill for our design. We can get the Net Cut closer to a balance with the fill by raising the retention pond design elevation up a little more. We'll use the exact same steps for material balancing to raise the surface model again.

1. Delete both the graded surface and the modified terrain.
2. Select the original design surface and this time set it to .08 m. and click the Offset Z button.
3. Again, regrade the surface design at 30 degrees and -30 degrees respectively.
4. Recalculate the Cut/Fill and our Net Cut is now .75 m³.

Our design is now well balanced in regard to material usage. We have enough dirt to create the fill from what was cut with only a little bit of excess material left over.

Export a Machine Model

We'll export a machine model so the retention pond design can be dug on-site via an excavator.

1. Ensure the completed design surface is enabled and showing in the Viewport.
2. Also ensure that no other design lines are showing.
3. Go to the Export tab and set the file Format to .xml.
4. Click Export Survey and name the file. 
5. Click OK to export the design to a chosen file location.

A Deeper Look: Detailed Analysis

We can get a detailed visual of the baseline terrain next to the graded design, using the Profile tool. 

1. Click on the Analysis tab.
2. Click on the Profile button.
3. Draw a profile line over the center of the retention pond.
4. Ensure both the Design and Baseline Timesteps are included in the Profile View.

We can also create a cross-section profile line to see how the topography will change after being built, from another angle.

As Built

Once the retention pond is built, we can fly and add the drone dataset as another Timestep for the as built and final product to compare the original design with the build. 

Use the Terrain Creator app to create a new timestep for a final look at the retention pond: 

1. After processing photogrammetry in the Terrain Creator app, click the To Existing button. 
2. The dataset is moved to the Virtual Surveyor app as a newly created Timestep.
3. The timestep is automatically created in the Timeline and is generated with the drone flight date as the Timestep name.

Extra: Calculate Water Volume 

• Want to know how much water the pond can contain? Check out this article to learn how to calculate water volume in the Virtual Surveyor app.

Credit

This article was created with the kind support of Mr. Humberto Moro. Thank you for sharing your knowledge and drone data.