Positioning accuracy

[Freddygang]

Hello everyone,
New joiner (and new drone owner) here.

I have a couple of questions about the DJI geolocation system.

I have to survey a small piece of land (about 2 acres) on an island in the Med. The survey should allow me to do the following
1) build an electronic 3d terrain model of the area and
2) give me views from set points at a specific height from the ground

I thought that a drone would be the ideal tool to achieve both.

I just ordered a heavily discounted mini 3 pro, and I'm doing some research on how to best achieve the 2 outcomes above.

My questions are really:

1) how accurate is the drone's geolocation system, and will I be able to position it at a set height and capture images
2) what is the best workflow to build a 3d terrain model

Please bear in mind that this is really a one-off exercise, so I am quite reluctant in investing huge sums in equipment or software packages

Thank you very much!

 

[slup]

1) how accurate is the drone's geolocation system, and will I be able to position it at a set height and capture images...

Hi there & welcome to the forum

You need to factor in that a civil GPS module like the one you have in DJI drones have an horizontal inaccuracy of approx. +/-5m. Furthermore, the height you see in the app is always relative the homepoint where the barometric sensor gets reset when the drone takes off... the drone in general, doesn't have a clue about height above ground in each & every moment during the flight besides very close to the ground where the visuell positioning sensors on the drones belly can work.

So you will not be able to use a drone for horizontal position... or height above ground pinpoint accuracy.

 

[Felix le Chat]

Hello everyone,
New joiner (and new drone owner) here.

I have a couple of questions about the DJI geolocation system.

I have to survey a small piece of land (about 2 acres) on an island in the Med. The survey should allow me to do the following
1) build an electronic 3d terrain model of the area and
2) give me views from set points at a specific height from the ground

I thought that a drone would be the ideal tool to achieve both.

I just ordered a heavily discounted mini 3 pro, and I'm doing some research on how to best achieve the 2 outcomes above.

My questions are really:

1) how accurate is the drone's geolocation system, and will I be able to position it at a set height and capture images
2) what is the best workflow to build a 3d terrain model

Please bear in mind that this is really a one-off exercise, so I am quite reluctant in investing huge sums in equipment or software packages

Thank you very much!

A good old-school accuracy workaround is to walk the site beforehand and drop GCP's right next to an obvious landscape feature, such as a large rock, corner of a wall/built structure and use existing WGS84 data to determine the centre of each point. After this: fly your grid capture deployment. The GCP's (Ground Control Points) are circles of white vinyl coloured in to form 4 black & white quadrants. You can cut these out of a roll of lino. Once you've stitched your 'orthomosaic' together, you have x-number of points visible on it that Google earth will give you the *accurate* northing and westing co-ordinates for.

 

[Dave Maine]

The GPS accuracy spec is written in such a way that the x y position is quite uncertain. The stated accuracy is + - 16 feet 90% of the time. In practice, you get around +- 8 feet often. This all varies by the hour, as the satellites used for the solution are always in movement, and the solution geometry is constantly changing.

You will need to take all this into account to decide whether your result will have any practical value.

 

[Freddygang]

A good old-school accuracy workaround is to walk the site beforehand and drop GCP's right next to an obvious landscape feature, such as a large rock, corner of a wall/built structure and use existing WGS84 data to determine the centre of each point. After this: fly your grid capture deployment. The GCP's (Ground Control Points) are circles of white vinyl coloured in to form 4 black & white quadrants. You can cut these out of a roll of lino. Once you've stitched your 'orthomosaic' together, you have x-number of points visible on it that Google earth will give you the *accurate* northing and westing co-ordinates for.

Thank you for the replies - the GCP's suggestion is a good one.

I am familiar with the gps inaccuracy, but how accurate are the drone sensor? E.g. if the barometric sensor is set at start, does it reliably stay constant and therefore will all the height measurements be consistent among themselves?

 

[slup]

...how accurate are the drone sensor? E.g. if the barometric sensor is set at start, does it reliably stay constant and therefore will all the height measurements be consistent among themselves?

You can expect a barometric height drift during a flight of approx. 3-6m... just compare the indicated height when you take off & when you land on the same spot again.

 

[crusen8]

I am afraid to tell you that ambient barometric pressure and temperature changes affect the measurements from the sensor. They are not scientific quality by any means.

 

[Glenh]

Hello everyone,
New joiner (and new drone owner) here.

I have a couple of questions about the DJI geolocation system.

I have to survey a small piece of land (about 2 acres) on an island in the Med. The survey should allow me to do the following
1) build an electronic 3d terrain model of the area and
2) give me views from set points at a specific height from the ground

I thought that a drone would be the ideal tool to achieve both.

I just ordered a heavily discounted mini 3 pro, and I'm doing some research on how to best achieve the 2 outcomes above.

My questions are really:

1) how accurate is the drone's geolocation system, and will I be able to position it at a set height and capture images
2) what is the best workflow to build a 3d terrain model

Please bear in mind that this is really a one-off exercise, so I am quite reluctant in investing huge sums in equipment or software packages

Thank you very much!

ve.

Hello
An interesting question. As you have limited funding (as do I) .

The main question - what exactly would you like to achieve? What results would you need? Things are relative.

The short answers from my experience:
1) The best that you can do with a Mini 2/3 series is work on relative (rather than definitive) positioning and heights. Ground control points are useful.
2) Check out the 'maps made easy' website. This will start you off in the realm of 3D modelling at a low cost.

 

[DougMcC]

All aviation cockpit altimeters are adjustable based on the CURRENT surface pressure at a known altitude. Before approaching an airport for landing the current reference pressure is broadcast and pilot sets that into the altimeter. If an airport is 1650 ft above sea level you want your altimeter to be accurate as you fly a pattern or approach at a known altitude above the surface - say 1000 feet above the runway. Your altimeter should shiow 2650 feet. That adjustment is made at every takeoff and landing and even in mid course. Surface air pressure changes all the time with temperature weather systems etc.

 

[Freddygang]

Hello
An interesting question. As you have limited funding (as do I) .

The main question - what exactly would you like to achieve? What results would you need? Things are relative.

The short answers from my experience:
1) The best that you can do with a Mini 2/3 series is work on relative (rather than definitive) positioning and heights. Ground control points are useful.
2) Check out the 'maps made easy' website. This will start you off in the realm of 3D modelling at a low cost.

Thank you very much Glehn.

In answer to your questions, ideally I would like to use the photographic data gathered via the drone to build a 3d model of the land, that I can use to produce as a base to produce some architectural 3d sketches in CAD. There is no need for the model to be georeferenced, but I would like the topography to be as consistent as possible (relatively to itself, rather than geographically correct). The other thing I would like to do is to position the drone where the top of the first and second stories of the building-to-be-designed will be (so let's say 4 meters and 8 meters from the ground), in various locations, to see how the views change at height. This is only to inform the design process, so it requires a lower level of accuracy (let's say that i'm ok with the drone being +/-5% out). I hope this clarifies things!

(And Maps Made Easy looks like it could be a great solution for the first part of my query)

cheers!

 

[rjwmorrell]

Hello everyone,
New joiner (and new drone owner) here.

I have a couple of questions about the DJI geolocation system.

I have to survey a small piece of land (about 2 acres) on an island in the Med. The survey should allow me to do the following
1) build an electronic 3d terrain model of the area and
2) give me views from set points at a specific height from the ground

I thought that a drone would be the ideal tool to achieve both.

I just ordered a heavily discounted mini 3 pro, and I'm doing some research on how to best achieve the 2 outcomes above.

My questions are really:

1) how accurate is the drone's geolocation system, and will I be able to position it at a set height and capture images
2) what is the best workflow to build a 3d terrain model

Please bear in mind that this is really a one-off exercise, so I am quite reluctant in investing huge sums in equipment or software packages

Thank you very much!

That may not be the tool for the job. If you need accurate mapping you may need an enterprise level drone such as a Matrice with RTK capabilities. Good luck, though.

 

[AMann]

All aviation cockpit altimeters are adjustable based on the CURRENT surface pressure at a known altitude. Before approaching an airport for landing the current reference pressure is broadcast and pilot sets that into the altimeter. If an airport is 1650 ft above sea level you want your altimeter to be accurate as you fly a pattern or approach at a known altitude above the surface - say 1000 feet above the runway. Your altimeter should shiow 2650 feet. That adjustment is made at every takeoff and landing and even in mid course. Surface air pressure changes all the time with temperature weather systems etc.

How do you set an altimeter accurately in flight? Are you getting it from ATC?

 

[DougMcC]

How do you set an altimeter accurately in flight? Are you getting it from ATC?

Yes they will announce something like “Gallup altimeter is two-niner . Eight two” and always from approach control. However the pilot is expected to have obtained an ATIS recorded updated info by listening to an fully updated automated airport conditions and ceiling which are called by an Alpha identifier. When calling a tower you would identify your plane and position and add something like “with KILO “. If that’s been updated you will get the more current and altimeter setting.

 

[EssenYVR]

All aviation cockpit altimeters are adjustable based on the CURRENT surface pressure at a known altitude. Before approaching an airport for landing the current reference pressure is broadcast and pilot sets that into the altimeter.

Altimeters are used to maintain separation between aircraft (typically 1000 feet for general aviation), but for that purpose they don't necessarily have to show an accurate altitude. And they don't, because the pressure gradient from the surface to flight level may not follow the standard profile that the altimeter is calibrated for.

What's important is that all aircraft are using the same measurement - which is essentially barometric pressure. This means that if my altimeter is reading 100 feet higher than the "true" altitude then your altimeter is reading that too, and as long as we're not flying toward each other at that same, "erroneous" altitude when we're OK.

That's not true of photogrammetry, of course.

 

[Droning on and on...]

Yes they will announce something like “Gallup altimeter is two-niner . Eight two” and always from approach control. However the pilot is expected to have obtained an ATIS recorded updated info by listening to an fully updated automated airport conditions and ceiling which are called by an Alpha identifier. When calling a tower you would identify your plane and position and add something like “with KILO “. If that’s been updated you will get the more current and altimeter setting.

The equivalent operation for a sUAV drone flight is setting the barometric altitude to zero on takeoff (automatically). This makes for a comparable accuracy to a manned aircraft in knowing a useful, calibrated altitude (above takeoff).