Blog | Data Alignment in Virtual CRASH

 

Updated: June 13, 2019

With the upcoming 2019 Summer Update, Virtual CRASH 4 users will have more ways to align data in Virtual CRASH. In this post, we’ll review the ways users can align aerial imagery, points, and point clouds. With this update, users will have the following methods to import and align data:

(1) Aligning Points with Aerials. Manual Image Scaling and Manual Alignment (VC4 legacy method, current VC3 method).

(2) Aligning Point clouds with Aerials. Manual Image Scaling and Manual Alignment (VC4 legacy method, current VC3 method).

(3) Aligning Points with Aerials. Manual Image Scaling and Manual Alignment with the Easy Alignment Tool using Control Points.

(4) Aligning Point clouds with Aerials. Manual Image Scaling and Manual Alignment with the Easy Alignment Tool using Control Points.

(5) Aligning Point clouds, points, and Aerials. Manual Image Scaling and Manual Alignment with the Easy Alignment Tool using Control Points.

(6) Aligning Point clouds, points, and Aerials. Automatic Image Scaling and Alignment with the Smart Alignment Tool using tfw files.

(7) Aligning objects using the reference point feature.

(8) Aligning points with Google Earth aerial imagery.

Each of these options is demonstrated below, where we want to align an orthomosaic and point cloud from Pix4D, as well as ground control points.

 


Data Source

Often, it is advantageous to shoot ground control points (GCPs) as part of the photogrammetry process in order to improve accuracy. The example data shown below was acquired during a field demonstration during the 2019 EDR Summit. The demonstration was lead by Andrew Klane of Forensic Mapping Solutions. In the photographs below, Andy Klane (in red) is using a RTK GPS device to measure locations of GCPs, as well as one test point. If you’re interested in drones, Pix4D, or RTK GPS devices, contact Forensic Mapping Solutions for help.

Here we see a table of the recorded GCPs. Note, these are in State Plane Coordinate System.

GCPsTable.png

With the GCP targets left in place (visible from above), a DJI Mavic Pro 2 drone was flown over the scene while photographs were captured.

The drone images were then brought into Pix4D. The GCPs were tagged and corresponding RTK GPS measurements were input into Pix4D. Pix4D then produced a point cloud and orthomosaic image of the test scene.

Picture1.jpg
 

Aligning the Data

1. Aligning Points with Aerials. Manual Image Scaling and Manual Alignment (VC4 legacy method, current VC3 method).

Importing and scaling aerial images is described in Chapter 9 of the User’s Guide. Once the image is imported and scaled using two fixed reference points of known distance, the point array can then be aligned with the image. The video below demonstrates the process.

 

2. Aligning Point clouds with Aerials. Manual Image Scaling and Manual Alignment (VC4 legacy method, current VC3 method).

Aligning point clouds and aerials can be done in the same manner as shown above. This process is demonstrated in the video below.

 

3. Aligning Points with Aerials. Manual Image Scaling and Manual Alignment with the Easy Alignment Tool using Control Points.

With the 2019 Summer Update, Virtual CRASH 4 users can align data using the Easy Alignment Tool. For points with aerials, simply lock the control grips to your reference points, and your image will automatically align and scale. This is shown in the video below.

 

4. Aligning Point clouds with Aerials. Manual Image Scaling and Manual Alignment with the Easy Alignment Tool using Control Points.

With the 2019 Summer Update, Virtual CRASH 4 users can align data using the Easy Alignment Tool. For points with aerials, simply lock the control grips to your reference points, and your image will automatically align and scale. This is shown in the video below.

 

5. Aligning Point clouds, points, and Aerials. Manual Image Scaling and Manual Alignment with the Easy Alignment Tool using Control Points.

If you have point array data you want aligned to your point cloud, or other 3D objects you want to align, just use the Easy Alignment Tool. Lock the control grips to reference point positions on the point cloud. This is shown in the video below.

 

6. Aligning Point clouds, points, and Aerials. Automatic Image Scaling and Alignment with the Smart Alignment tool using tfw files.

If you use photogrammetry software, such as Pix4D, then you’ll love the Smart Alignment Tool (released with the 2019 Summer Software Update). Just load your tfw file and all of your data will import into your scene already aligned! This is shown below. NOTE: If you import both a point array AND a point cloud using the Smart Alignment Tool, the z position of the point array is determined FIRST such that the lowest point in the point array will sit on the x-y plane. The z position of the point cloud object is then adjusted to be in agreement with the point array. This may result in point cloud points being beneath the x-y plane. If this occurs, simply select both the point array and point cloud objects simultaneously and adjust the z positions using the z control grip (press “3” on your keyboard), or select both the point array and point cloud objects and change reference point z value until all point cloud points are above the x-y plane. This will maintain the relative z-alignment of your point array and point cloud. In a future update the user will be given the option to first position the point cloud such that all points are above the x-y plane, with the point array being positioned relative to the point cloud in z.

Learn more about reference points >

Below is another example of using the Smart Alignment Tool on data collected at the 2018 IPTM Symposium on Traffic Safety Crash Test #3.

 

7. Aligning objects using the reference point feature.

You’ll notice that images, point clouds, and point arrays now have a new “reference point” menu visible in the left-side control panel. This is what is used by the Smart Alignment Tool to automatically align data. This allows separate objects to be aligned to a common point in space. For example, here we see two halves of a point cloud. The upper half has been separated from the lower half in Cloud Compare. This is one way to maximize memory allocation by inputting the trees and buildings as a separate point cloud object, allowing the greater visible density. The lower half will be used for generating the terrain surface with the Easy Surface Builder. Here we see our two point clouds.

When a point cloud (or point array) is imported into Virtual CRASH, the full dataset translated to a position typically near (0,0). Since these are two halves of the same point cloud, the data within each point cloud file is with respect to the same common coordinate axes. To align the data in Virtual CRASH, simply use the exact same reference point (x,y,z) position for both point cloud objects. In the example below, we’ve set the reference point for both point cloud objects to (0,0,-10). This brings the point clouds into proper alignment.

In this case, the Easy Surface Builder tool can be used on the lower half of the point cloud.

 

8. Aligning points with Google Earth Imagery

To align your point array from total stations or RTK GPS devices with your Google Earth imagery, simply load your points and your Google Earth images.

Enable point snapping. Switch the point array to “Select, Move and Align”. Use the local translation control grip to snap the translation control grip to a fix reference point. Here we’ll use a utility pole that was shot with the total station. Once the translation control grip is snapped to a reference point in the point array, left-click on the translation grip and move the control grip to hover over the corresponding location on the Google Earth image.

Next, use the local translation grip to move the point array’s yaw control grip to a second reference point. Here we use a point shot on the white lane line. Left-click on the yaw control grip and move the yaw control grip to the corresponding position on the Google Earth image. Here we’ll rotate the point array until this second point sits on top of the white lane line.

Now your point array is aligned to the Google Earth image.