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Author Topic: Is it possible to build a before and after DEM?  (Read 2155 times)

jessica_tay

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Is it possible to build a before and after DEM?
« on: July 27, 2017, 07:39:45 PM »
Hello,

i have taken images of a site before and after the flowering season. And have aligned the photos in separate chunks.

I will probably be using the same GCP for both chunks.

Is it possible to build a DEM for the before and after flowering and is it accurate to see the height of the flowering plants?

Thanks!

Jessica

SAV

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Re: Is it possible to build a before and after DEM?
« Reply #1 on: July 28, 2017, 04:20:48 AM »
Hi Jessica,

It will mainly depend on your ground sampling distance (GSD), the accuracy of your GCPs, the height/height difference of the flowers and image overlap.
For example, if you have a ground sampling distance of 10cm/pixel and your flowers are only 10cm high, you probably won't see much.

But generally it is possible if the survey settings have been chosen properly.

Regards
SAV

jessica_tay

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Re: Is it possible to build a before and after DEM?
« Reply #2 on: July 31, 2017, 01:00:11 PM »
Hello SAV,

Thank you for the reply.

I have another question. I have a before flowering photos and I have markers on it. And I build a DEM out of it.

Then I export the markers and import it to the after photos, but the markers are 'floating'. Do you have any idea why and how do I solve this? I thought that by during the same GCP, i will be able to build an after DEM to see the growth. I've attached some images.

or is there other methods that I can do this?

Thanks for you help!

jessica

jessica_tay

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Re: Is it possible to build a before and after DEM?
« Reply #3 on: July 31, 2017, 04:29:13 PM »
The points appear to be floating because is it still belonging to the projections of the before photo (top photo).

Is there a way to export the position of the markers and add it to another chunk? (i added it to the chunk that is below but this problem occurred).


Thanks!

SAV

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Re: Is it possible to build a before and after DEM?
« Reply #4 on: August 02, 2017, 05:11:17 AM »
Hi jessica_tay,

You can easily export markers from a previous project/chunk using the SAVE function in the reference pane (icon with a small floppy disk). However, after importing and using them in a new project/chunk you will have to 'manually link' them to your new pictures/model again if you want to use them as control points. PhotoScan needs to know where these 'old markers' are located in the new images/model to be able to make use of them.

You could try to employ coded markers instead which might work in your case. PhotoScan is able to automatically detect them, hence no 'manual linkage' (as mentioned before) would be necessary. Note that the size of the coded markers needs to be adjusted to your project's ground sampling distance (GSD), as already mentioned/discussed in this forum.

All the best.

Regards
SAV