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Messages - stihl

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1
General / Re: Aligning Aerial Photos of Forest Canopy
« on: October 25, 2019, 05:06:58 PM »
Hi James,
I've always gotten much better results with dense vegetated areas when using downsampled images. This is due to there being less pixel variance on lower resolutions on vegetation which causes Photoscan to find more reliable tie-points and thus is able to align the images. This also accounts for the densified point cloud.
If you change the accuracy parameter (from High to Medium or even Low) then Photoscan will first downsample the input images before finding tie-points.
In the case that this does not give suficient results then it's also worth while to manually reduce the size further in an external application and then using these lower res images again (with high, medium and or low parameter tries).

I've gotten very good results from a nearly useless data set in Nigeria by downsampling the input images in Photoshop before bringing them back in to Photoscan and running it on Low quality dense cloud.

3
General / Re: Effects of the quality of dense cloud generation
« on: June 19, 2019, 05:42:42 PM »
Changing the quality level from High to Medium to Low changes the resolution of the input images.
If you select Medium, Photoscan will first downscale the images to 1/4th of their original resolution. 1/4th of the original pixels available results in a dense point cloud with 1/4th of the amount of points when compared to processing the project on High.

Per Image Side Downsample Factor                               Per Image scaling            Expected point density
Ultra High Quality:                          1:1                                      1:1                                             GSD        1:1
High Quality                                        1:2                                      1:4                                             GSD        1:2
Medium Quality                                1:4                                      1:16                                          GSD        1:4
Low Quality                                         1:8                                      1:64                                          GSD        1:8
Lowest Quality                                  1:16                                   1:256                                       GSD        1:16

Example: If Ultra high produces 100 million points, Medium quality will then create 25 million points.
If your project GSD is 2 cm/pixel, your maximum DEM resolution at Ultra high will be 2 cm/pixel. At Medium it will be 8 cm/pixel.

If Photoscan has to work with lower resolution images to create the dense cloud (and geometry of objects), then this will have an impact on the quality of the reconstructed geometry. It's best to make a small bounding box in your project area and test different quality levels to see if the low quality parameter provides you with the level of detail you're looking for.

4
- accuracy of 0.001 mm doesn't seem to reasonable value selected, as the nodes of the printed checkerboard pattern cannot be measured with such precision (I would suggest to use 0.1 mm max),
Can you please explain what changing this value in metashape actually has an impact on in the generation of sparse / dense cloud process? Does this affect the SIFT algorithm somehow or the bundle adjustment? How does having a grosely wrong accuracy value come into play in the reconstrution?
The accuracy of the markers directly impacts the 'weight' of the manually placed markers. If a manual marker is slightly off from the actual position then this introduces a misalignment of the images due to the weight of the markers being higher than the weight of the tie-points found by Photoscan.
The alignment determines the location and orientation of each aligned image. When building a dense cloud, an algorithm is used to create depth maps from stereo paired images. If the images are slightly misaligned then this will introduce noise in the final point cloud.

5
You will need to use the optimization tool to refine the camera calibration parameters. As currently there are significant lens distortions which affect your products.

6
Hi szhu0,

If you already have a DEM TIFF, why do you want to merge it with the XYZ data? From what I understood these two files are of the same project/data set? If so then there's no need to use the XYZ. The DEM (elevation) geoTIFF file will have the same kind of information, if it's in the same resolution.

You can assign a hillshade and colour ramp to your DEM TIFF in ArcGIS by doing this:
http://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/displaying-a-dem-with-hillshading.htm
or
http://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/displaying-a-dem-as-a-shaded-relief.htm

7
General / Re: Dense Cloud Quality
« on: February 28, 2019, 06:43:48 PM »
 -Photo Resolution Scaling:  Alignment Quality-
Highest:        2:1
High:               1:1
Medium:       1:2
Low:                1:4
Lowest:         1:8


-Photo Resolution Scaling:  Dense Cloud Quality-
Ultra High:      1:1
High:                   1:2
Medium:           1:4
Low:                    1:8
Lowest:             1:16


Example: If Ultra high produces 100 million points, Medium quality will then create 25 million points.
If your project GSD is 2 cm/pixel, your maximum DEM resolution at Ultra high will be 2 cm/pixel. At Medium it will be 8 cm/pixel.

8
General / Re: inaccurate volume
« on: November 12, 2018, 06:40:15 PM »
If you have not scaled your project with either markers (GCPS) or a sale bar then your model will have *no* value other than looking pretty.

Photoscan has no idea what the size of the box is unless it's scaled. Otherwise it can be a box the size of ..a box, or a box the size of a house.

9
General / Re: Increase processing performance by a factor of 2
« on: November 06, 2018, 12:03:48 PM »
Interesting read. The first thing comes to mind; is the processed data (before you split it in 4 chunks) not still active in the cache or working memory?

When I'm running only one instance of PS I can see all of my cores being utilized, each and every time. And I definitely see a slowdown when running two unrelated projects simultaneously.

Curious to see what Agisoft has as an explanation.

10
General / Re: DJI Phantom 4 RTK
« on: October 18, 2018, 10:47:33 AM »
Regardless whether this particular RTK setup is stable or not - like 'aerial_survey' mentioned it's still mandatory to perform the measurements of control points scattered throughout your project site. If you have nothing to check your data against to it's pretty much useless from a surveyor perspective. In other words, you'll still be spending time walking around your project site and measuring points with RTK-GPS.

Furthermore it's interesting to think whether you'll even want to use RTK for the drone positioning and not just stick to PPK which has numerous advantages.

11
General / Re: Mesh VS DEM orthomosaic
« on: October 03, 2018, 12:06:32 PM »
In previous versions of Photoscan there was no option to create a DEM, only to create a Mesh. The Mesh and DEM are similar in that they are both models from the interpolated Dense Cloud. If you're unsure if you need a Mesh for your products simply stay with creating DEMs.

Using a DEM will sometimes yield better results when creating True Orthomosaics (an Orthomosaic based on a DEM and not a DTM) near building edges than using a mesh. This is because a DEM is interpolated using IDW whereas a Mesh is using linear interpolation to create a model.

12
General / Re: How does quality of dense cloud influence the resolution?
« on: October 03, 2018, 12:01:26 PM »
The DEM is a direct product from an interpolation of the dense point cloud.
If you've selected medium quality for the dense point cloud, you'll end up with a point density 4 times lower than is possible at the highest setting.
This means that the interpolated DEM will be of a lower resolution than your project GSD, which directly correlates to the used camera and flight altitude.

The Orthomosaic is generated by projecting the undistorted Ortho photos either onto the DEM or another available mesh surface. Since they are being projected, the resolution of the DEM (or surface) is irrelevant.

13
General / Re: Automate Bounding Box - Large Scene/Terrain
« on: September 07, 2018, 12:11:59 PM »
Hey Lauren_O,

That's quite simple. Once your images are aligned, go to the Toolbar > Tools > Reset Region.

This will re-align the bounding box ground plane (the red side of the bounding box) with the determined ground plane of the 3D model.

14
General / Re: Your opinion on USGS Agisoft Processing Workflow
« on: August 31, 2018, 05:31:26 PM »

Funny, seeing how I followed this workflow and got worse results than following our own developed workflow.

I suppose it all depends on your data set.

Could you outline your workflow and describe your type of data, out of interest?
Unfortunately I prefer not to go into depths of our workflow. I can say that it's quite different than what USGS following for their gradual selection filtering.
The results that the USGS workflow yielded were slightly worse absolute errors compared to independent check points than our own workflow and showed a decrease in the amount of detail for the dense cloud. Most likely due to larger misaligment errors compared to our own workflow.

15
General / Re: Balancing dense cloud specs vs mesh specs
« on: August 31, 2018, 05:29:06 PM »
Jwoods that's very much correct, unless Agisoft changed this parameter during an update.

If you're running out of RAM during the Build Mesh stage and you're unable to add more RAM or virtual memory then it'd best to duplicate the chunk (for safety) and rerun the dense cloud at a lower quality level. From what I remember, going down one step in the quality level means a factor 4 reduction of generated dense points. With this info you can get a rough sense of the amount points you'll end up with after the dense cloud stage.

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