Why LIDAR for Volumetrics?
Author: Lewis Graham
Published On: June 18, 2021
True View EVO, based on our widely-used GeoCue LP360 software, is bundled with every True View 3D Imaging Sensor (3DIS), including our subscription programs. True View EVO contains all of the workflow tools (including embedded Applanix POSPac drivers) for taking True View 3DIS data from the USB data stick containing raw observation data to a gorgeous, geocoded 3D colorized point cloud. But this is just the beginning for many projects.
While the colorized point cloud is a fantastic visualization tool, most projects aim to produce derivedproducts. No worries - True View EVO contains a plethora of tools for creating a wide variety of products from point cloud data. In a previous issue, we took a detailed look at the volumetric analysis tools in True View EVO/LP360. In this issue, I want to take a step back and discuss why you would use a LIDAR system rather than photogrammetry for this type work.
EVO Tools for Volumetric Analysis
Volumetric analysis within a point cloud setting is the process of using the point cloud to compute the volume of material. True View EVO contains an advanced set of volumetric analysis tools that allow for a variety of computational scenarios such as:
- Simple stockpile with base defined by a "toe"
- Simple stockpile with overhead features such as an overhanging conveyor (that require removal from the computation)
- Stockpiles with a priori base level (for example, a surface is defined by a survey performed before the stockpiles were placed)
- Volumes with bases defined by geometric shapes (e.g. material contained in bins)
- Borrow pit analysis
- Change analysis over time such as cut and fill computations
Volumetric Case Example
As an reminder, let's take a look at a simple situation where we want to measure the volume of a stockpile that has an overhanging conveyor.
This scene, collected with a True View 410 3D Imaging System (3DIS®), is depicted in Figure 1. Note the useful display modes in True View EVO that make the situation with the stockpile easy to visualize. I have set the display to render by Triangulated Irregular Network (TIN) and shade by elevation color bands. Notice how we can very clearly see the stockpile as well as the overhead conveyor.
Figure 1: Stockpile with Overhanging Conveyor
Computing the Pile in EVO
The process of computing the volume of this pile will follow a few simple steps:
- Create a stockpile "toe"
- Change the classification of the conveyor points so they will not be included in the stockpile volume calculation
- Classify any extraneous "noise" points - this is often required if the point cloud was photogrammetrically derived. It is seldom needed with True View 3DIS LIDAR data
- Compute the volume
True View EVO has a robust set of tools for manually and automatically digitizing stockpile toes. In fact, the automatic stockpile toe creation tool includes an option to detect and classify non-stockpile overhead features.
If the overhead structure is separated from the stockpile, the classification is "clean" and you will not have to do any additional work. If the overhead structure touches the pile (e.g. a conveyor at pile level), you may have to do some manual cleanup. This is very straightforward using the variety of manual point cloud classification tools within True View EVO.
When to use Photogrammetry vs. LIDAR?
The question naturally arises as to when to use photogrammetry and when to use LIDAR. Of course, if you already own a True View 3DIS, this is a moot point since our 3DIS systems do both. However, the real question here is when does one need to advance from pure photogrammetry systems (e.g. a Phantom 4 RTK) to LIDAR?
One obvious need are stockpiles that have been overgrown with vegetation. Photogrammetric point clouds "float" at the top of vegetation, causing the computed volume to be larger than reality. The extent of the error depends on the relative size of the pile compared to the height of the vegetation.
Coarse, Undulating Surfaces
A second common need are materials that have very coarse, undulating surfaces. Photogrammetry typically gives a rolling approximation to these surfaces. A good example (see Figure 2) is measuring the volume in log yards (feeder stock for pulp mills).
Figure 2: Stockpile Of LogsOther common examples include:
- Dark surfaces that are hard for image correlators to do point matching. A good example is stockpiles of coal
- In general, coarse materials such as riprap
- Overburden estimation for mining since the overburden is often vegetation covered
- Cut and fill computations prior to grubbing
- Complex situation when a narrow channel exists. These channels are usually entirely missed by photogrammetry since it is hard to get multiple camera views in these locations. A good example is monitoring berms in tank farms.
True View 3DIS Will Do BOTH!
If you have a True View 3DIS, you can address all of the various volumetric scenarios since you have both photogrammetry and LIDAR. Of course, this does not mean that photogrammetry only is not a good solution. You just have to be careful to restrict it to those scenarios where photogrammetry shines. Regardless of your choice in sensor technology, we can help you with the software and technical support tools needed to be successful.