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Scaling Maps

Pnovotny

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A client asked to map a business compound but needs the map to be scaled to 1:1000 & 1:10000 (metric).

I'm using a M2EA with an RTK module.

DJI Pilot indicates that the GSD is 1.39 cm per pixel at 50 meters.

Can anyone help me understand how do I determine the necessary height I need to fly at for the scale to match?

Please let me know if any additional data is needed.

Thanks for your help!
 

Airmapper

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I second @Gagey52

Altitude decision is based on max resolution desired or operational necessity to clear obstacles.

Setting to a specific scale is done in post processing.

However, with some knowledge of final desired scale, you could make decisions to fly higher for faster coverage of the area or vice versa for detail. That would take a bit of experience with your equipment, if it were me I'd maybe do some print previews of an old known dataset at those scales and see if the detail at a given altitude is overkill at that size or not, with that info you might could decide what altitude to try knowing you wouldn't degrade the end result or collect way more detail than needed.

I'm sure you could do some rough estimates with math, but you would likely want to get a bit better detail than you need rather than lower.
 

jaja6009

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This is not in your question but why the Mavic 2 Enterprise Advanced unless this is a thermal project.
The RTK module does not take the accurate coordinates and place them as the principal point in each image's metadata like a Phantom 4 RTK or Mavic 3 E would. I guess if you use the RTK module you will at least have the elevations of the aircraft correct in terms of positioning, but be sure to change the elevations manually as this accurate information will not be what's in the image's metadata for altitude/elevation.
The 48MP RGB images are large and take more time to process than a Phantom 4 Pro or RTK 20MP.
I have made various maps with this drone and, well to be blunt, they sucked in terms of accuracy and were the most inaccurate of any drone I have ever mapped with.
 

Pnovotny

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This is not in your question but why the Mavic 2 Enterprise Advanced unless this is a thermal projec
Thank you for the important information, which I'll have to look into.
This isn't a thermal project, it's the only RTK enabled aircraft I own.
Aside from it I have the Dual, Air 2S and Mini 2 and this is my first attempt at a mapping project.
 

AMann

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I assume your client was asking for a 1:1000 image scale- a very high quality image, which has to do with image size scale on the film or sensor and it affects the image resolution.

Map scale is entirely different and is the ratio between how much you enlarge the final print in post to how much ground area it covers.

There is a formula for calculating image scale for aerial photography-

S=H/f

Scale = Height above ground /Lens Focal Length

To solve for height:

H=S x f

So, using a 9mm fixed focal length lens, you need to convert the focal length to the same units as your height - in this case feet:

f9mm = 0.0295276 ft

H=1000 x 0.0295276

Height = 29.53 ft

Now that will give you a straight 1:1000 scale on the negative (if using a film camera) or on the sensor.

So at a 1:1000 scale, flying at 29.5 ft AGL, using a 9mm lens, your little 1/2” sensor (6.4mm wide) will cover:

0.252” x 1000 = 252 inches or: 252/12=

about 21 ft on the ground.

But remember- This will only be the image scale that you are getting for each image on the sensor, your final map scale will depend on how much you enlarge or reduce it in post and when your client prints or displays it.
 
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jaja6009

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I assume your client was asking for a 1:1000 image scale- a very high quality image, which has to do with image size scale on the film or sensor and it affects the image resolution.

Map scale is entirely different and is the ratio between how much you enlarge the final print in post to how much ground area it covers.

There is a formula for calculating image scale for aerial photography-

S=H/f

Scale = Height above ground /Lens Focal Length

To solve for height:

H=S x f

So, using a 9mm fixed focal length lens, you need to convert the focal length to the same units as your height - in this case feet:

f9mm = 0.0295276 ft

H=1000 x 0.0295276

Height = 29.53 ft

Now that will give you a straight 1:1000 scale on the negative (if using a film camera) or on the sensor.

So at a 1:1000 scale, flying at 29.5 ft AGL, using a 9mm lens, your little 1/2” sensor (6.4mm wide) will cover:

0.252” x 1000 = 252 inches or: 252/12=

about 21 ft on the ground.

But remember- This will only be the image scale that you are getting for each image on the sensor, your final map scale will depend on how much you enlarge or reduce it in post and when your client prints or displays it.
He is outputting a map or orthophoto so I would guess the client was saying map scale.

Sorry if I am misunderstanding, but you want him to fly at 21' AGL? That would take an extremely long time for even small areas and DJI Pilot will not allow you to fly that low in photogrammetry image acqusition missions.
But that is some cool information though.
 
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AMann

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He is outputting a map or orthophoto so I would guess the client was saying map scale.

Sorry if I am misunderstanding, but you want him to fly at 21' AGL? That would take an extremely long time for even small areas and DJI Pilot will not allow you to fly that low in photogrammetry image acqusition missions.
But that is some cool information though.
21 feet is low, but at 1:1000 it’s a good image scale for mapping kangaroo rat burrows and making 3D models of bedrock milling sites - ask me about those research projects sometime.

As for map scale, it does not depend on the flying height, it’s the graphic output size one chooses to print a map on paper or to zoom in on a display. Of course, one has to take image scale into account for this to ensure good resolution. To be able to print a map at 300 dpi, one has to know both the image and map scale and what size the map product is going to be in order to do it.

there are a lot of references about this online, but I‘ll put together a short guide on how to do all this and will post it here separately.
 

Pnovotny

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Thank you all.
Unfortunately I have a bit of a fever at the moment so focusing is out of the question... but will review once I feel better.
 

Gagey52

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21 feet is low, but at 1:1000 it’s a good image scale for mapping kangaroo rat burrows and making 3D models of bedrock milling sites - ask me about those research projects sometime.

As for map scale, it does not depend on the flying height, it’s the graphic output size one chooses to print a map on paper or to zoom in on a display. Of course, one has to take image scale into account for this to ensure good resolution. To be able to print a map at 300 dpi, one has to know both the image and map scale and what size the map product is going to be in order to do it.

there are a lot of references about this online, but I‘ll put together a short guide on how to do all this and will post it here separately.
Thanks for the info, look forward to seeing your guide. 👍
Regards
 

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