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How Many GCPs, what overlap / sidelap, and can I use two drones?

I need to generate a topographic survey of a 700 acre site (basically rectangular shape) where the maximum vertical accuracy error will be one foot (12 inches - 30cm).  My understanding is vertical accuracy should be within 3X GSD, so that means my GSD needs to be 10cm (4 inches) or better.  So I will be flying at max AGL (400 feet). There’s going to be fairly large elevation changes over that large of an area probably as much as 200 feet during some flights (maybe only 25 feet on other sections).  The terrain is hilly but without water/trees/crops - somewhat desert like and sparse structures.  The absolute accuracy on the horizontal plane also can have an error up to 12 inches.  Given these requirements, what would be a good number of GCPs to use (how do you determine that figure?) and what would be a good minimum sidelap / overlap?  

Also wondering could two people using two identical drones, such as a DJI Phantom 4, could be used to save time or would possible variations in the altitude measurements between the two drones possibly introduce large errors in the resulting work product?

 

Hi Jeff,

See my input below:
 
1. Given these requirements, what would be a good number of GCPs to use (how do you determine that figure?)

For an area of similar size (that was flat) we noticed that adding more than 5 to 8 Ground Control Points (GCPs) did not improve the accuracy much more, i.e. the time investment of adding GCPs would not result in a large improvement of accuracy. As your area is hilly, I would add a few more GCPs to cover low and high areas. Make sure that the GCPs are evenly spread and are inside the area of interest so that many images see the GCPs. This makes sure that they have a better impact on the project. If I were you I would measure a few more GCPs that can be used as checkpoints to verify the accuracy of the results. 

I can recommend this article from the Knowledge Base: https://support.pix4d.com/hc/en-us/articles/202557489

2. What would be a good minimum sidelap / overlap?  

The general recommendations for overlap are minimum 75% frontal and 60% side overlap. However, if your area is desert like, I would increase this minimum to 85% frontal and 75% side overlap. There is more information on flight plans in this article: https://support.pix4d.com/hc/en-us/articles/202557459#label1

I would try to reduce the height differences during the flight and in any case make sure that the Ground Sampling Distance (GSD) at one place is not more than twice the GSD at another place. Otherwise, there could be issues during the matching step of the initial processing. More on this in these two articles:

3. Also wondering could two people using two identical drones, such as a DJI Phantom 4, could be used to save time or would possible variations in the altitude measurements between the two drones possibly introduce large errors in the resulting work product?

It is possible to use two drones for a single project. As long as you have GCPs in your project there should not be an issue due to the image geolocation. Make sure that you have enough overlap between the flights of both drones and that there are not too much differences in the environment (e.g. shadows,…). This could result in multiple layers in the point cloud or other effects which would require additional manual work. For such a large area it may be of advantage to use a fixed wing drone, which has a longer flight time. 

 

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In general the more overlap you have, the closer the accuracy is to 1X GSD in any direction.

Every project is different so 75% or 95% overlap might be required so my advice is to adjust the overlap to the maximum for the time onsite and batteries you have available.

Personally I would look at using RTK instead of spending the money on manually surveying GCPs but that is an already well debated topic in the industry…

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Well using RTK is great - but given his drohne (DJI Phantom4) it is not that easy to get RTK on this Drohne as far as i´m concerned

 

Back to Topic.

 

I already did such big Projects maybe i can give some tips to help u.

 

Plan the flights in the office (especially if u use 2 drones), it is not as stressfull and you can coordinate better between the 2 drohnes. As long as they use the same camera u should be find with your results too. otherwise it could get complicated to determine the quality of your results (different GSD for different cameras)

 

The more GCP´s the better - i cannot say this often enough if u invest 1 or 2 hours into getting very good and some more GCP´s it is much better than recognizing later that you dont have enough (happend to me twice and it was atroucios)

 

Another Thing which was already mentiond bevore. If you are not sure that there are enough structurs for Pix4D to find (desert as you said) choose a higher overlap than u would normally -  depending on your time and the size I used to do a small area where i tested the percentage of the overlaps first and then did the big project. Could still save you some time 95% overlap takes ages on huge areas. (but i did an area of 1.5km² where we installed a new fish ladder and they needed it ASAP so booking a plane was not an Option)

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