Tutorial Pix4d [PDF]

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Post-Processing for drone mapping ——MAPT



Kristof Van Oost Emilien Aldana Jague He Zhang



25/4/2018



Sections:



1. 2. 3. 4.



Drone-mapping workflow RTK and PPK How to use RTKlib Cloud and DSM generation in Pix4D



Drone Mapping Workflow



Process Capture



Analyze



Tie Points and Overlap



CREATING A MOSAIC BY STITCHING SEVERAL ORTHORECTIFIED IMAGES. SOURCE: (MEO ET AL, 2012).



Figure 1. Using a tie point to represent a feature



A tie point (TP) is a feature that you can clearly identify in two or more images and that you can select as a reference point, as shown Figure 1. Figure 2. Tie points in overlapping images



The perspective center coordinate provides a strong geometric tie between the models, helping recognize adjacent images.



RTK and PPK Non-RTK drone



-with an onboard GPS receiver -Normally, you will encounter an inaccuracy up to several meters, especially in Z directions.



In that case, we need ground control point (GCP) to help improve positional accuracy in processing.



RTK and PPK RTK drone -Connect to a base station or a virtual reference station (VRS) -Camera positions are calculated in real-time, up to 2 to 3 centimetres of accuracy



Post-Processing (PPK) -In some cases, direct connection is not available -We can perform the computation referring the ground reference afterward with a PPK drone system



Setup EUREF Permanent GNSS Network Corrections From Base Station



Drone with Reach Kit



Figure. Distribution of EUREF in Belgium



•Base stations is a Reach unit in Wi-Fi AP mode, configured Reach RS as NTRIP. Mount Point:BRUX00BEL0



Data management



RTKlib For Post-Processing, we use RTKlib



For visualization For converting UBX to OBS and NAV file



For post-processing



RTKlib Step 1. Start RTKLIB RTKCONV after downloading raw files from Reach to your PC.



RTKlib



Step 2-1. Start RTKLIB RTKPOST



from Rover from Base from Rover



First configure the options



RTKlib



Step 2-2. Set Base Position



Open the LLH. File with RTK_Plot, find the Fix point.



Single Float Fix



We can get it from “Solution file” in RTKPlot



RTKlib Step 2-3. Set Setting1 and Setting2



Personal experience: Due to signal quality, if you use different settings, you may acquire different positional output.



RTKlib



Elevation mask



Integer Ambiguity Res



RTKlib Step 3. Check the position reliability using RTKPLOT A shift between “fix and hold” and “continuous” [False Fix]



After processing, we got POS file and event.POS file which can be open with RTKPlot. We compare the positions from different settings, if they are all close (within centimeter), meaning that the position is trustworthy. In this case, we use “Fix and Hold”. Otherwise, if the positions vary from different settings (meter level), “Continuous” is more trustworthy.



RTKlib Step 4. Arrange Coordinate Info in NotePad++ and Excel



Open events_POS file with NotePad++, check the number of recordings (whether match photo numbers).



Note that: the first recording may be the “switch on” of camera. Distinguish by time interval.



Q: quality 1:fix 2:float 5:single



RTKlib Step 4. Arrange Coordinate Info in NotePad++ and Excel



~22cm Remember to remove the antenna height of the drone.



Save as CSV file



RTKlib More information about Reach manual, go to: https://docs.emlid.com/reach/common/tutorials/gps-post-processing/



More information about RTKlib, there is a manual:



Geotag the photos in R (optional) Put photos and coordinate.csv and exiftool.exe in a folder.



Write GPS info into JPG files.



Processing in Pix4D Now we have geotagged Photos, we could process in Pix4D or Agisoft Photoscan.



Step 1. start Pix4D, create new project



Next page



Processing in Pix4D



Processing in Pix4D



If we haven’t geotagged the photos, we need to import the coordinate from file.



It’s the liberty you give to the software, that to what extent it can trust the coordinate of photos. There is an positional adjustment through calculation.



We can set 0.05 m here. Next page



Processing in Pix4D



Next page



Processing in Pix4D Here we use 3D Maps



Processing in Pix4D Step 2. Select images and GCPs



Select images (remove images: during taking off and landing, flurry, turning)



Adding GCPs. Here we use GCP only as Check Point. Note: choose a proper Geo System



Processing in Pix4D Step 2. Select images and GCPs For images



For GCPs



GCP or CP ? Here we use CP



Remember to set a proper Coordinate System



Uncheck the images you don’t use



Processing in Pix4D Step 3. Configure Processing Options



Uncheck (for saving time)



uncheck



Processing in Pix4D



uncheck



Processing in Pix4D Step 4. Run Initial Processing



Now we can run the processing: 1. Initial Processing. After that, we will have a rayCloud view. Then we can select GCPs, mark the location in the images. Note that: this step is for calibration with GCP or validation with CP. If we are going to use direct georeferencing (without GCP survey), we can skip this and directly run all the 3 steps.



Processing in Pix4D Step 5. Marking GCP and reoptimize



As for marking on the images, we select the GCP and click its position on the images, After several clicks, we could do it by automatic Marks. We make sure each mark is correct, then we click Apply. When finish the marking, we do “reoptimize” in “Process” menu.



Processing in Pix4D Step 6. Run Following Processing



Now we can run: 2. Point Cloud and Mesh and 3. DSM, Orthomosaic and Index After that, we will have generated Cloud and DSM.



Processing in Pix4D Now we can do what we want. Eg. Cloudcompare ArcGIS



Figure. Topography change between two survey



(a)



Before plowing (b)



Reference cloud



After plowing Figure. Outputs of cloud-to-cloud (M3C2) of reference cloud and (a) cloud before plowing (b) cloud after plowing