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Author Topic: DEM From Mesh or Dense Cloud  (Read 6110 times)

droscovi

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DEM From Mesh or Dense Cloud
« on: May 18, 2016, 10:41:30 AM »
Hello,

I am a long time user of Photoscan for mapping and measurements in a mining environment. Prior to the last few updates I would create my dense cloud, mesh (height field) and then export my DEM. Now I notice there is an option to create my DEM from dense cloud OR mesh. Is there any difference in quality between creating a DEM from mesh or dense cloud?

Also, while on the topic, I find that a mesh from height field gives me a good result but creating a arbitrary mesh almost seems to have more detail but takes significantly longer. For my purposes with measuring pile and cut/fill volumes, is there any reason why an arbitrary mesh would be preferable over a height field or is the height field sufficient?

Thanks!

stihl

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Re: DEM From Mesh or Dense Cloud
« Reply #1 on: May 18, 2016, 11:08:19 AM »
The difference between generating a DEM directly from the Dense Cloud or the Mesh is the way Photoscan interpolates the data from a point cloud to a solid model.

When you choose to build a DEM based on the Dense Cloud, Photoscan will generate a DEM on the specified resolution, build from the the dense points directly with an "Inverse Distance Weighting" (IDW) interpolation.

The DEM resolution that is specified by Photoscan is directly related to the Quality mode that was chosen when building the Dense Cloud. Where a higher quality level means a higher density 'Dense Point Cloud' and thus a higher resolution that's available when building a DEM (without upsampling the data).

When you choose Mesh, the points get interpolated as a "Triangulated Irregular Network" (TIN) where it linearly interpolates the Dense Points as small triangles. The higher the specified resolution is, the smaller the triangles are.

Both are fine interpolation methods, but for surveying purposes I'd advice to use IDW. This method also usually generates less noise around building and structural edges.

An Arbitrary Mesh is when Photoscan tries to interpolates all the dense points through a TIN method. Suitable for when you have a Dense Cloud of a statue or an object.

When you're dealing with stockpiles, you would only need a Heightfield Mesh.
This is when Photoscan produces a TIN from only one viewing angle; the one that's perpendicular to the red lined side (the underside) of the bounding box.
If you have your Point Cloud geo-referenced you can always reset the bounding box. This will automatically align the underside facing the earth for when it's not correctly placed when wanting to create a Heightfield Mesh.

For creating a model of a stockpile, you would only really need the highest Z point at every XY location, therefor a Heightfield Mesh is sufficient to produce an accurate volume if you've flown a proper nadir aerial grid over the stockpile with decent overlap.


« Last Edit: August 10, 2016, 02:40:03 PM by stihl »

ajam13

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Re: DEM From Mesh or Dense Cloud
« Reply #2 on: August 10, 2016, 01:43:48 PM »
Dear All,

Stihl, thanks for nice and clear explanation!

Two questions refering to Stihl's text:
This is when Photoscan produces a TIN from only one viewing angle, the one that's perpendicular to the red square from the bounding box. If you have your point cloud geo-referenced you can always reset the bounding box accordingly (it will automatically rotate  the bounding box so that the the red square is facing the earth) when it's not already set up right.

1. How can I "reset the bounding box" so the red plane coresponds to my georeferenced dataset (this is to XY plane)?
2. In case of semi vertical wall that has many objects "hanging out" (reinforced wall with many bolts), would I get a better model (so that bolt constructions would be really model as a square attached to the wall), if I rotate the red plane of bounding box into e.g. XZ/YZ plane and using "heightField Mesh"?

Thanks for the answer in advance!

lg, Maja

stihl

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Re: DEM From Mesh or Dense Cloud
« Reply #3 on: August 10, 2016, 02:04:46 PM »
Hi Maja,

You can easily reset the bounding box to match the geo-reference automatically by going to Tools in the menubar and selecting "Reset Region".
You will likely get the best results by resetting the region first and then using a heightfield mesh.

However, if the bolts that are attached to the wall are too thick, the underside of the attachment wouldn't get meshed as this is out of view for Photoscan when the wall is looked at from directly above.
Then you should choose an arbitrary mesh to generate meshed data for occluded areas  when viewed from above. This will generate a more complete model. Keep in mind that an arbitray mesh requires large amounts of RAM data when the project consists of many images. More info on RAM requirements here.
« Last Edit: August 10, 2016, 02:09:36 PM by stihl »

ajam13

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Re: DEM From Mesh or Dense Cloud
« Reply #4 on: August 10, 2016, 05:26:51 PM »
Hi Stihl,

Thanks for fast respond! :D

going to Tools in the menubar and selecting "Reset Region".

Oh, as easy as that ;)! Great to know.

quote author=stihl link=topic=5357.msg27889#msg27889 date=1470827086]
However, if the bolts that are attached to the wall are too thick, the underside of the attachment wouldn't get meshed as this is out of view for Photoscan when the wall is looked at from directly above.
Then you should choose an arbitrary mesh to generate meshed data for occluded areas  when viewed from above. This will generate a more complete model.
[/quote]

Here are the prints screens of the "bolts construction" on the vertical wall; DPC / mesh model: https://www.dropbox.com/sh/8q2ttgzthwv5on2/AAAClPgqXPPba2unuYldXKS5a?dl=0

Since the photos were taken from a bit lower point, I actually have many points on "bolts construction" downside. But sure, they are not meshed correctly using "Height Field" and yes the option "Arbitrary" takes ages (I stoped the process).

Thus, my question is, if it would work (the meshing), if I rotate the bounding box, so that the red plane is ll with my semi-vertical wall and then chose "height field"?

Thx!

Rainy & cold greets,
Maja

stihl

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Re: DEM From Mesh or Dense Cloud
« Reply #5 on: August 19, 2016, 11:59:42 AM »
Hi Maja,

After looking at your screenshots, your assumption is correct if you'd rotate the bounding box so that the red plane is parallel with the semi vertical wall.
If you then choose heightfield it will build the mesh as if the vertical wall is your 'floor' and the bolts are sticking out vertically.

ajam13

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Re: DEM From Mesh or Dense Cloud
« Reply #6 on: August 19, 2016, 12:56:26 PM »
Hi Stihl,

Oh that is a great option .. to model vertical surfaces (acquired from ground) in a fast "height field" mode  :D

I thought somehow that the red plane of the box defines or refers to the coordinate system .. so should be ll to the xy plane of used coordinate system ::).

Thanks for the enlightenment  ;)!

bw,m


outis79

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Re: DEM From Mesh or Dense Cloud
« Reply #7 on: September 01, 2016, 06:01:16 PM »
Hi all,
sorry Stihl, isn't the Poisson Surface Reconstruction used for building the Arbitrary Mesh? Can you explain please.
Thanks 
 

stihl

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Re: DEM From Mesh or Dense Cloud
« Reply #8 on: September 01, 2016, 07:24:44 PM »
Hi outis79,

That could very well be. I wish I had more insight into exactly what alghoritmes are being used for which step. 
Then again I can understand if such information is kept privately as it might help competitors.