Messages

76

General / Can A Project Be Scaled Using Distances Between Points of Reference?

« on: January 21, 2018, 11:07:51 PM »

In the absence of having geo coordinates of ground control points, can you improve the relative measurement accuracy by providing known distance measurements between points?  I know this is possible with some software I used before (Pix4)  you provide the software with the real measured size of some feature (i.e. length of a building) and, it will scale the project accordingly, which should increase the relative accuracy.

Is there a workflow in Photoscan to do this also?

77

General / Re: Do large differences in GSD screw up processing?

« on: January 13, 2018, 08:45:25 PM »

DJI Phantom 4 Pro Drone. 

78

General / Criss Cross Obliques With Less Overlap Versus One Direction?

« on: January 12, 2018, 11:33:31 PM »

I'm doing a topographic survey of a rural area with about 500ft of elevation change (I'm breaking it up into sections where each section has only 250ft of elevation change).  Will output a DXF with contour lines and possibly a DEM or DTM.  I only need accuracy within 12 inches.  Will have ground control points.

Obviously when you are generating output for 3D features, it's more accurate to also have obliques.  One can see the crispness of the structures and lack of blobbing/noise that the use of obliques contributes.

Given the accuracy requirements and that this is mainly for land features, not structures, I am wondering the following:

(a) In general you want 1/3 GSD so 4in/pixel for 12-inch accuracy, that's low, I will be twice better GSD.  And I'll have 80/70 overlap which is also pretty decent.  In that case, are obliques in reality a waste of time?

(b) If I did do obliques, what would produce better results, run a mission in one direction with 80/70 frontlap/sidelap, or run two missions in two different directions (criss cross) but with 50/40 overlap/sidelap.  In other words is having more overlap in one perspective more valuable than having two different perspective obliques with less overlap?  Again this is for land topography measurement, not 3D modeling of structures.

79

General / Do large differences in GSD screw up processing?

« on: January 12, 2018, 08:55:45 PM »

I've read that for a large site where the topography has large changes in elevation, if you fly the same altitude AGL from the starting point, it can create issues in processing.

However, if you have a target GSD, and therefore a target altitude AGL, as long as you never exceed that altitude so that the variation in GSD will always be better GSD than the target, it would seem to me that shouldn't cause any issue.  The issue would be if the GSD was a lot worse than the target at certain points in the flight path.  Or is the potential issue in processing just any big difference in GSD despite whether it better or worse resolution?

Example:  I want to generate a topographic map (DEM/DSM) where I want the elevation measurements to be accurate within 12 inches.  It is widely stated the elevation measurement are *typically* accurate within 3 times the GSD.  So I need a GSD of 4inches/pixel.  (In reality it will be better because that GSD allows for an altitude above 400ft).  So if some of the photos have a GSD of 1inch/pixel because of high points on the site being surveyed, whereas others will be 4inch/pixel at the low points of the site, is that going to create a problem in processing?

80

General / Re: Agisoft lens

« on: December 21, 2017, 08:12:08 AM »

So I've been reading about camera calibration for a UAV and there seems to be fairly little information out there.

I'm going to be doing a survey flying at 400ft.  I'm going to have ground control points laid out for me by a land surveyor.  With enough ground control points I read you can disable using georeferencing information in the photo EXIF.  Does this also mean camera calibration does not provide significant benefit? The accuracy level I need is just 1-foot or better.  If camera calibration is necessary has someone documented the proper steps for this type of application?

81

General / Re: Work Flow for GCP

« on: December 16, 2017, 11:05:53 PM »

I’m looking at doing a large top survey of 700 acres and I need absolute position accuracy within 10 inches (25 cm.) It’s widely accepted then you need a GSD of 1/3 that. I’ll have better due to altitude limitations unless I launch from a hill. Let’s say I’ll have 5cm/pixel GSD.

There seems to be no definitive information out there regarding the number of GCPs I would need to insure the geolocation data of the resulting work product my geolocation data will be accurate to within 25cm.  Even in SAV's great workflow post use see:

9. Assuming that you have a SUFFICIENT/b] number of high accuracy ground control points...

How do you determine what is a "sufficient" number of GCPs?  I can't find any guidelines anywhere on the Internet after exhaustive searches!

For all I know, with my accuracy requirements, maybe all I need is one GCP as an absolute coordinate "anchor" so I can later conver to the local coordinate system.  A big question is, would it be necessary for me to adjust (I don't use the word 'correct') the geolocation data in the EXIF information of the photos by noting the difference in X Y and Z coordinates of the drone with the GCP and run a script on the photos to get them more in line with the GCP?

My other low-cost strategy would be to buy an Emlid Reach RS, just one, and set it at 5-10 GCPs for a few minutes and log the data. I could then use a free or low-cost subscription to a CORS network and perform PPK to correct the coordinates of the GCPs in post processing.

So the million dollar question is essentially how do you determine the number of GCPs you need to be able to disregard the geolocation data in the photos and still achieve the accuracy you want? 

82

General / Large Area Topo - How Many GCPs? Overlap? Obliques?

« on: December 16, 2017, 04:09:52 AM »

I'm looking to do a topographic survey of a 700 acre site where the vertical measurement accuracy needs to be within 12 inches.  So dividing that by 3 means I can have a GSD of 4in/pixel or better (it will be better due to altitude limits.)  So I have a few questions.

There seems to be no information anywhere regarding what determines the number of ground control points you need.  I HAVE found conclusions that it's the law of FAST diminishing returns.  Most example use 3-10 ground control points but those have been for site sizes in the 20-300 acre range.  The site I am looking at doing is about 40% flat and 60% has hills with lots of elevation change up to about 500ft.  It's a desert-like landscape so there are sparse buildings and vegetation, roads, but not forest or water.

The other question is, given the objectives, what's a good minimum frontlap and sidelap to shoot for?  I was thinking 75/65. 

And finally, given the site characteristics, is shooting a set of oblique images in addition to the nadir images a waste of time or can it significantly improve the vertical measurement accuracy?  I'm using a DJI X4s camera which is 9mm focal length.  I'm guessing given my accuracy requirement not being that high, and a wide angle camera, the obliques would probably just double my data size for little benefit but I'm not sure.

83

General / Re: Camera Elevation AGL Ground Control Points Above Sea Level

« on: December 16, 2017, 04:08:43 AM »

So I'm getting ready to do a topographic survey.  Vertical accuracy needs to be within 12 inches so GSD will be 4inches/pixel or better (will be better due to altitude limits)  It's a large area, 700 acres.  Hoping 7 GCPs will be enough.  The elevation data for the GCPs laid down by a land surveyor will obviously be more accurate than the drone. 

I'm wondering if let's say the GSD is going to be 3in/pixel.  Frontlap 75 probably and sidelap 65, is it going to be worth  noting the ASL elevation at the launch point right next to a GCP and then modifying all the EXIF elevation data or just uncheck the photos and only use the data from the GCPs when processing in photoscan.  I'm a coder (PHP) with some Python experience but if it's really not going to give me much benefit I'd just assume go the simpler route.

84

General / Re: Contour Lines in DXF

« on: December 13, 2017, 08:55:31 PM »

Is there a setting somewhere to specify how close together you want the contour lines drawn?

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General / Contour Lines in DXF

« on: December 13, 2017, 03:57:48 AM »

I have a request to provide a topographic map in the form of a DXF file with contour lines every foot for a 1,500 acre area.  I believe this is for import into AutoCAD (I think the Civil 3D version I'm guessing at this point - still more info to come.)  Can the DXF file generated by Photoscan be used for this or do the contour lines need to be generated first by an intermidiary GIS software (like QGIS) or generated within Civil 3D itself.  Essentially, if someone is asking for DXF format that can generate contour lines, can I just give them the DXF straight out of Photoscan?  If not, what other processing do I need to do?

86

General / List of all possible export file formats?

« on: November 29, 2017, 09:49:10 PM »

is there a list anywhere of all possible export file format in Photoscan Pro?

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General / Camera Elevation AGL Ground Control Points Above Sea Level

« on: June 28, 2017, 02:17:08 PM »

So the photos from my Phantom 4, the camera elevation is Above Ground Level (AGL).  The 5 ground control points I had surveyed use Above Sea Level (ASL). 

Is this going to screw things up?