Elevation Accurate

Hello, i’m new here in forum, and i did some flights with DD. But I generate the Elevation tool after upload the photos, and I noticed that it make some wrong readings. The terrain is very flat, with only 1 m elev maybe the difference between some points. But it showed like 10 meters of difference. It does a checklist or tip to make it better, or can make this error normal?

Hi @allygemelli,

Welcome to the DroneDeploy Forum! The errors you’re experience are to be expected due to GPS errors inherent to the drone. I suggest taking a look at the following articles to learn more about accuracy and see how you can improve the accuracy of your maps:

  1. How Accurate is My Map?
  2. Capturing Ground Control Points (GCPs)

Cheers,
Christina

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If you are using a DJI Phantom 4 Pro drone, the altitude data in a picture’s geotag does not come from the GPS receiver in the drone. Rather it comes from estimating the altitude using the barometer reading. I believe DJI made this choice in order to greatly reduce the variation in the measured altitude so the drone will fly more level. If I look at all the altitude data from a 10 min mission with 100’s of pictures, the recorded altitude is constant within 20 cm without the first and last pictures. Actual GPS altitude readings typically vary by several meters over a 10 minute period (land your drone and take pictures over a 10 min period and then look at the longitude and latitude GPS values in the geotags - you will see they cause the location to move around by meters and GPS altitude measurements are worse).
So while the altitude data can show little variation during a DD mission, it’s absolute accuracy is terrible and can vary by 900’ over the course of a year. Whenever the barometer changes due to weather (up to 1" Hg over a year period in our area), the altitude recorded will change. Thus elevation values measured in a DD 2D map created with data from a P4P drone may not be accurate but the relative differences should be much better.
Using GCP’s is certainly one fix for this issue but there are easier ways to improve the absolute elevation accuracy.

I’m not concerned with the actual elevation being accurate for what i’m doing. I’m trying to get the low spots in our flat fields, where water will collect to help with placing drain tile. Flying P4P. With that being said, what would be your recommendation?

Sounds like your need can be accomplished just by running a standard mission. The color elevation map will show you the low (blue) areas. You can adjust the sliders to see more contrast. From there you can place location markers to get the coordinates for filed layout.

Hello,

I am using a Phantom 4 Pro and the horizontal accuracy is good enough for our requirements however the verical accuracy is unusable. It is ~60 meters out. How can i improve this without using GCPs? I am interested to hear your ideas as I do not have access to highly accurate surveying equipment.

Thank you

There is now an elevation adjustment capability in DroneDeploy. You just need to know the elevation at one spot on the map and then use DD to measure the elevation of that spot. You then enter the difference into DD elevation offset tool to move the whole map. Now your elevation measurements and contours display and the like will show absolute elevation values. This does not fix any relative elevation differences across your map. For those you need GCP’s on the ground distributed across your site before you fly your mission and then you need to input the GPS coordinates of the GCPs to DD when you process your photos. But you only need to use GCP’s if you need cm-level accuracy for measurements on the map. I typically get 4" elevation accuracy across 300’ distance from my missions flown on calm, overcast days after offsetting the map elevation so it is correct at one spot on my map.

Hope this helps.

Regards,
Terry.

If there is existing monumentation (paving, manholes, land features, etc) you can create GCPs in this site. The elevations are within 1m. This will at least take most of the camera error out of the flight, but also gives you a base network to tie you flights together.

https://www.gps-coordinates.net

*EDIT I meant 1m, not foot. The site is in Lat/Lon/Elev (m).

Hey guys,

Thank you.

I will only have my mobile phone or garmin GPS to capture lat, long and elevation for GCPs and i understand this will reduce the accuracy of the survey? Or do you suggest i use the values from that website link?

I would like to make the elevation values as accurate as possible but think the only way is to use the values at the homepoint (using my phone…) and make the whole site relative to these values? Many GCPs with the mobile with be too inaccurate?

Thank you

Maybe you could get the GPS of the takeoff point from Google Earth Pro. This may be more accurate than your phone.

I was able to find the GPS coordinates for my takeoff point from Google and found it to agree with the value on a detailed LiDAR map covering my site.

Regards,
Terry.

For any site you need AT LEAST 4 (preferably 5) solid points with 5 or more tagged images to be sure you have something good. By using the website I provided your absolute elevations will be within 1m, but your site will be much more relatively accurate . To get even more accurate X,Y fly the site first then make points in QGIS. If possible, put your GCPs down beforehand. This will be 10x more accurate than the site I provided or Google Earth. Problem is you have to have access to the site and make two trips (or wait around for a couple hours) because Live Map is not accurate enough for this

Can you provide a KMZ polygon of the subject? I’d like to see what you are working with.

Hi guys,

I could put some markers down for GCPs but the only way i can get the x,y and z is with my phone or Garmin GPS unit so im not sure if they will be accurate enough?

I dont need the Z to be cm accurate just better than i currently get. The DSM is showing 60m but real world is 117m. Perhaps this is due to my homepoint being near tall (20m) trees or a limitation with GPS.

Perhaps i can use the ‘elevation offset tool’ and add the home point elevation from a topo map if i can find one.

Thank you!

@JamesMitchell The GPS on your phone will not be accurate enough to capture your data. You will need to trimble unit. I would take a look at our Accuracy Support Document

Emlid Reach RS+ also works well.

The elevation offset you are seeing is not due to your home being near 20m tall trees. It is due to atmospheric variations that the barometric pressure sensor in your drone is sensing. To get better than 60m you do not need GCP’s. Just use the elevation offset feature of DD and set the elevation to the known elevation of a point on your map. This will easily get you to within a meter of the absolute altitude assuming you can find the elevation of the takeoff spot on a topo map. Otherwise use your cellphone as it will be within 10m and better if you average the readings over time. If you need better accuracy, then GCP’s are the way to go. For myself, I get sub-foot accuracy for the elevations around the takeoff point by offsetting my map using data from a LiDAR bare-earth model available for my area.

Regards,
Terry.

You might want to invest in a “BadElf” handheld GPS unit for around $650. You can find this init on the Internet; and it’s horizontal accuracy is typically better than 1m without post-processing. It’s vertical accuracy is pretty good; and it’s much more stable than your phone.
I’ve used it for several years with good results.

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