Jun 18,2017

What Is Survey-Grade Accuracy in Close-Range Photogrammetry Software, and Why It Can’t Be Achieved With the Point Cloud Technology?

In this post, we will analyze the term "survey-grade accuracy" and compare two common close-range photogrammetry technologies commonly used in the market, highlighting their benefits and disadvantages.

 

Drone Surveying Accuracy Datumate 

 

 

 

 

 

 

 

 

 

 

 

Datumate’s photogrammetry software is based on the Image Plane technology. Image Plane workflow is completely different from other (Point-Cloud-based) solutions available on the market. The image above illustrates the difference between the technologies when it comes to accurate measurements.

Point Cloud Technology

On the left side of the image, you can see the Point Cloud technology, which is widely used today by most of the photogrammetry companies on the market.  The basic product of the Point Cloud technology is Texturized Point Cloud and it provides a nice visualization of the surveyed area.

Object Distortion and Lack of Survey-Grade Accuracy

If you have an experience working with Point Clouds, you probably know that the Point Cloud model will most probably be distorted in some areas. You are likely to run into some problems if you try to extract data from Point Cloud model or generate vectors.  For example, you can see in the image above, that we can’t see precisely where the rails lay, so we need to guess where the edge of the vector is. This is a distortion that cannot be fixed, as this Point Cloud has been generated automatically.

We typically need to extract data from Point Cloud by guessing, in this case, we lose the accuracy and can’t “guesstimate” correctly, meaning, we can’t assume that each vector point is 5 cm or 10 cm accurate. Also, in this case, some of the image elements can’t be viewed properly.  Those who require electric lines inspection, for example, wouldn’t be able to see them at all on Point Cloud Model.

Image-Plane Technology

Now, let’s describe Datumate’s approach, which is based on Image-Plane technology. Datumate software does generate texture and point clouds for visualization, but these outputs are completely separate from vectors generation. We can generate vectors before we generate Point Cloud models, and thus we don’t need to wait for the end of the image processing step.  With Datumate’s workflow, one can pick any of the captured images, choose the object of interest and start digitizing. The digitizing will be done on the same chosen image, with the ability to zoom-in up to 1 pixel and see the object of interest as clear as the raw image and begin drawing. One can draw lines, points or polygons on different levels.

Survey-Grade Accuracy

Once we draw a vector and mark a specific point on one of the images, this same point is then automatically detected in all the overlapping images. The vector is then projected on any of the images that we select, enabling us to review the digitized information.

Behind the scenes, Datumate’s software engine will detect the drawn points in other overlapping images. In the “Approve” step the user can review the estimated accuracy per each specific point. The user also has a manual control of the accuracy review. In the areas where it’s hard for image processing algorithm to detect the accuracy, the user can edit the automatic detection. For objects such as electric lines, the epipolar line display helps the user to place the points accurately on the object. With this approach we use basic photogrammetric principles to extract the 3D coordinates of each point, detect the object on the pixel level and also review the result.  This unique workflow is what enables the survey-grade accuracy of Datumate's solutions.

 

Survey-Grade Acuracy - Point Cloud vs Vectors Close-Range Photogrammetry Datumate

 

 

 

 

 

 

 

 

 

 

Point Cloud vs. Image Plane – Summary

To sum up all of the above:

  1. One can reach a higher accuracy (survey-grade accuracy) on a pixel level than with the Point Cloud model.
  2. The user can validate the accuracy in the “Approve” step of DatuSurvey™ , while the accuracy of the Point Cloud cannot be reviewed.
  3. The image-plane technology is quite easy to use as is the Point Cloud technology.
  4. As opposed to Point Cloud, you don’t need a lengthy image processing to begin digitizing the area with image plane technology.
  5. In terms of outputs, Point Cloud technology provides beautiful visualizations and 3D models. The image plane technology is ideal for high-precision map generations, that ultimately the engineers can sign off on.  With Datumate’s image-plane technology, export to DXF is straightforward and can generate data in layers which simplifies final map generation in any CAD software

 

Got questions about survey-grade accuracy? Would like to see the online demo of Datumate's close-range photogrammetry solutions? Click here to contact our geospatial professionals.