I have several doubts related to volume calculation
1.Does we need GCP foe more accurate volume determination ? and Normal geottaged images in geographical cooradinates is enough for reliable results between 90-95% of accuracy which gives better result ?
2.Does trail version of pix4d gives wrong result compared to actual volume of the pile.
3.I have calculated volume in pix4d and global mapper it gives large difference in both the results?
4.Give me any suggesion i need to achieve 90-95% of accuracy in volume calculation.what are the steps i want to follow
If geotagged images are used and the volume measurements are supposed to be computed for small areas, GCPs are not required for accurate volume measurements. However, if volumes have to be calculated for large areas, GCPs are required for accurate measurements.
When calculating the volumes, the vertices of a base have to be marked in at least two images in order to calculate the error. Additionally, it is important to select the right settings for the base that fit your purpose. Step by step instructions how to draw a volume can be found in the following article 202560319 and description of settings can be found here 208652926.
If the same base is used in both Pix4Dmapper and Global Mapper for volume measurement, the difference in values should be minor. It is possible to export the base from Pix4Dmapper which can be imported to Global Mapper. For more information: 211245943.
To achieve accurate volume measurements we recommend to have high overlap between images, sharp images, well distributed GCPs, Manual Tie Points (MTPs), etc.
GCP’s are always welcome in a project no matter the size of it. Not having GCP’s would impact differently a project where you have RTK or PPK rather than in a regular GPS information project. To better understand the accuracy of the outputs please see: 202558889
You will have to understand what exactly is your comparison baseline. Do you compare two volumes for two projects without GCP’s or you compare one with a GCP’s vs. no GCP’s etc…
I am flying a 50 acre stock yard that has several piles ranging from 2000 cubic yards to 20,000 cubic yards, flying it with a matrice 200 with a zenmuse x5s focal length 15mm. I have 5 permanent ground control points surveyed in with a Leica GS14. The gcp’s are just around the perimeter of the stockyard at varying elevations. I have surveyed the piles in by hand at very tight intervals and my volumes are coming back 10-13% off. When I check the elevations in point cloud the toe of the piles are usually spot on with the Leica survey but the top of pile elevations are anywhere from 2-3 feet off. I have checked and double checked my camera setting, I am getting great pixelated pictures with no blur or rolling shutter and the ortho comes back clear. I have tried almost everything I can think of, any suggestions or thoughts on these issues? Thanks
After reading the description of the issue, I could not see any major issues in the workflow you follow. The “top of pile elevations are off” could be related to the way the dataset was acquired.
The DJI camera has the rolling shutter that can be an issue for the reconstruction depending on the flight conditions ( low height of the flight, high speed of the drone ) and the area that is mapped. My go-to solution in these cases would be to:
Capture the project area again while flying slower and/or higher to reduce the amount of linear rolling shutter distortion that is present in your images.
Could you give it a try with the linear rolling shutter enabled and let me know if this solves the issue?
Hi Charanya, including GCPs in a project does not depend on the area, GCPs are welcome if you want accuracy in the project no matter what the size is. What is the accuracy of the GPS you have? If your volume calculation area is small, then the error due to not having GCPs will be lower, if the volume area is more, the error will accumulate.
Hi,
So the gps have an accuracy of +/- 5m. What sort of an error accumulation can I expect if I dont use GCPs, can you give me a ball mark figure per 1 acres area.
Thank you
There are several factors to consider, so I wouldn’t be able to give you a range. I can give you some numbers that might help in your calculation. The relative accuracy of the outputs in general and without GCP should be expected to be 1-3 GSD. The absolute accuracy of the outputs should be expected to be 1-2 pixels (GSD) horizontally (X,Y coordinates) and 2-3 pixels (GSD) vertically (Z coordinate).
For instance, for a project of GSD of 2 cm, the horizontal accuracy expected is about 2-4 cm and the vertical one 4-6 cm.
If the reconstruction is good (good overlap between the pictures, etc…), you can also think of using scale constraint (3 for example). You can find how to add the scale constraint here: https://support.pix4d.com/hc/en-us/articles/205360375
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