Photoscan Markers

[ekbmuts]

In shooting a limited area, approximately 150 meters x 150 meters, where I am interested in the ground plane and what's on it (as opposed to trees and other elements that project up from the ground):

a.  Does it matter where I place my Photoscan markers as long as they are visible to multiple cameras?

b.  Does it matter how many Photoscan markers I use? 

c.  Does it matter which Photoscan markers I use? There are a lot to choose from.

d.  With Photoscan markers placed, do I need any other measuring device to appear in my shots, such as a 3 meter long stick or anything like that?  Or does Photoscan automatically recognize its own markers and also know how far apart the markings on them are and thus scale the scene correctly?

By "Photoscan markers" we are obviously talking about the black and white ones that you can print from Photoscan.

And finally, I've read in other posts that there is a command in Photoscan to detect markers in photos automatically.  Where exactly is that command located?

Any input would be appreciated.

Jon

[Milderinne]

Hello everybody,
I think the topic and related questions from ekbmuts are very interesting. I found this post when I was searching for more information about markers and especially experiences from other (advanced) users or software developer ;-)
So, does anyone has any thoughts about that?
I would be happy, if anyone could share their knowledge about that important topic!
Kind regards!

[stihl]

As a general rule of the thumb:

A) Placement is very important. Use GCPs (markers) on the corners of your project site and GCPs in the middle so that the model doesn't concave.
Imagine your model is made of vinyl and the GCPs are poles that are keeping it leveled. Use GCPs to support the model.

B) The amount of GCPs will have a huge impact on your absolute accuracy. The amount you'll need depends on the size of the terrain and how much local elevation changes there are. Depending whether you use precise camera coordinates or not, I suggest to use at least 15 GCPs and at least the same amount of check points that aren't used during the processing.
You need these independant checkpoints to check your model for its absolute accuracy. The photoscan processing report alone will not suffice for this.

C) I only use our own made GCP markers. These work great in all conditions. Can't comment on the automated Photoscan markers

D) Photoscan can recognize it's own markers. However if these points were not measured in the field with some sort of measuring device (total station or GPS device) then these points serve no use to geo-reference the project.
As an alternative you can measure the horizontal distance between the two markers you placed in the field and then use this as a scale bar in photoscan. Mind you; this will not geo-reference your model. You'll need GCPs  with coordinates attached.

[Art]

Hi
This is not quite on the same topic but how do I get PS to recognise non coded markers
cheers

[stihl]

Non coded markers can be automatically detected by following this part of the manual:

http://www.agisoft.com/pdf/photoscan-pro_1_2_en.pdf#page=53&zoom=auto,-304,616