ASM/REM 404 - Lab7: Orthophotos and Orthomosaics

[Music] all right welcome back this is lab seven for our class and uh pretty short lab today we're going to go over how to create orthomosaics and explore some options with those and look at how we export the ortho photos themselves from meta shape and play around with those a little bit so i've opened my lab6 document from last week and the very first thing that we want to do is uh give ourselves a bit of an insurance policy here and save this to a lab 7 file that way we don't mess anything up all right so i've got my canopy model from last time here remember we did three things a surface model a train model on a canopy model and uh let's do a couple things first so let's flip over and look at our dense point cloud uh that we created and uh i'm just going to double check the region that we that we created just to make sure that it's looking good like i wanted to it hasn't changed at all still got our our area in it okay that looks pretty good because it it's only going to create the ortho within this region of course it only created the dem within the region anyway so the ortho will be constrained to that now i want to open up my surface model here and we're going to go up to work flow and choose build orthomosaic and a couple things to point out okay so one is that this is going to build the orthomosaic off of whatever the active dem is and so make sure that you have the right one selected we're going to use the surface model in this case okay and for the options for building the orthomosaic you can see the projection information is grayed out and the reason it's grayed out is that we've already established the projection with the dem and so it's not going to let us change our mind at this point if you want it in something different you've got to go back and redo the dem and then you can use that to to ortho rectify your your photos for the parameters so we're going to use our dem again the active dem as our surface if we had built a mesh model here then we would have the option of using that mesh model for the for our surface for the orthomosaic 2. the blending modes refer to how metashape is going to adjust the colors in the overlap regions or along the seam lines of the orthophotos and the mosaic option here just does a dodging color dodging sort of technique there's a average which just takes the average pixel values for the overlapped areas and then disabled of course just turns that off so we'll leave that as our default with mosaic it's going to produce a set of seam lines which tell us sort of like which image the uh you know set of pixels comes from right and and those will be sort of like sharps you know straight uh lines and some geometric shapes if you wanted something or needed something that was a little bit more irregular or maybe natural then you can turn this refined seam lines option on enable hole filling if your dem has voids or holes in it this is a good option to to turn on and have it fill those in so that you don't get blank spaces in your orthomosaic pixel size this is set off of the original images and so we can change this you know you may want to have this be in this case like one and a half centimeters rather than 1.4 and change right a lot of times we would want to uh have this match another data set that we would uh have okay uh the region stuff we've already set that um we don't need to worry about setting that here okay so when you've uh got all these options set the way you want them you can just hit okay and let it rip this is going to take a little bit of time so i'm going to pause here and then we'll come back when it's done all right that's finished running now and we can see it shows up here is the ortho mosaic in the workspace pane if i double click on that it's going to open it up in the in the same window that the terrain model was in and i've got the seam lines are already turned on that was going to be the next step but we'll go ahead and look at that right now so the seam lines are just the the the poly lines that show where it sort of switched from using pixels from one image to pixels from another image and you can turn those on and off with this button up here on the main toolbar and just toggle them on and off let me turn my markers off so they're not in the way either and you know it's just kind of kind of curious it's worth looking at these just to uh to see sometimes you'll see patterning on the image that's the result of these seam lines too so let's go ahead and turn these off and and just kind of look around at this image you know this has turned out really pretty well and i just love looking at these orthomosaics i think they're really cool um and you know we can we can zoom in and see a number of things like here's our uh base station for our rtk unit and the and sort of the rover unit that we had right we could see our ground control points it did a really nice job at sort of reconstructing this uh this woodland patch here i'm going to turn these seam lines back on this isn't in the lab so much it's just something i wanted to point out see all these little tiny uh polygons with the uh with the seam lines so this is basically meaning that that it's pulling pixels from one image versus another and it's just it's like pretty uh kind of squirrely around here where it's getting these images from and sometimes if these get really bad then it creates this smudging effect or these weird artifacts in your orthomosaics and this can be due to having a surface model that's really sort of overly detailed for the purposes of your orthomosaic and so if you're seeing a lot of these or if you're getting really kind of lousy reconstruction in these spots then you may want to try creating a dem at a lower coarser resolution and then use that as the basis for the ortho mosaic in in this case it looks like it did a reasonably good job so i'm not terribly worried about it um so uh yeah it's just kind of fun spend a little bit of time playing around with this ortho mosaic and checking it out when you're ready then the next thing that we're going to do is we want to duplicate this orthomosaic okay and it's going to take a second to do that you can rename it i'm not going to worry about renaming it at this point but what we want to do after this is finished here is uh now re-do our orthomosaic so we're going to do build orthomosaic again and it's going to overwrite the active sort orthomosaic and this time we're going to turn the blending mode off so choose disabled here and go ahead and run this this is going to take a little bit of time i'll pause again and then we'll come back and then look at what effect the blending mode has on the on the final product all right so here is the ortho mosaic that has the color balancing turned off and uh you can see some pretty obvious uh differences here uh between these two if i zoom in and just sort of toggle back and forth with these right there's the color balanced one and there's the one with the color balancing turned off okay so you can obviously see the uh the seam lines here okay so uh i think it's worth using this as an example just to uh kind of point out that some of the patterns that we may see uh that appear to be sort of like color differences or ground sort of patterns may actually be artifacts or products of this the differences in the color between the images and then the color balancing process itself okay so just something to bear uh in mind so um next thing we want to look at here is just like how do we get these images and products out of meta shape you know metashape is really like a pretty cool application and can do a lot of stuff but you know it's not really uh a gis or an analysis application and so a lot of times we need to get these products out and then put them into another product like arcgis or an image classification package right and so to do that we're going to come over here in the workspace pane and i'm going to right click on my ortho mosaic and i'm going to choose export and we'll just choose this first option to export it as an image and we've got a whole bunch of options here so the first is that you do have the option of choosing a different coordinate system if you want to export in that again i strongly recommend that you choose a projected coordinate system and uh you know reprojecting raster data in other software applications can be kind of a hassle and metashape does it very efficiently so it may be a good idea to have meta shape exported in the projection you want rather than dumping it out and then trying to reproject it someplace else so you can change your pixel size here this split in blocks option is really useful if you have a really large uh ortho mosaic you know a big orthomosaic can easily be you know several gigabytes in size which becomes a real bear to kind of deal with and so with uh with this option then it will split it into tiles or blocks of uh you know in this case 10 000 by 10 000 pixels and so the individual images themselves are just much smaller and they're easier to deal with okay background color you know whether you want that to be white or black the region boundaries if you wanted a just a subset of what your ortho mosaic was you can you can set that in here and then just some options about you know sort of creating the files and compression that's applied so once you get all these set the way you want them you can just go ahead and hit export we're not actually going to do that right at the moment i just wanted to point out that this is how you would you would do that so i'm just going to cancel out of this at this point and so for the lab the next thing we want to do is actually create a quality report so go to export a a report you know hopefully you remember how to do that from the previous labs but uh so i won't go through that here but you will need to submit that report uh with your uh with your final um write up for the lab okay so the last thing we want to do is look at the actual ortho photos themselves and uh and metashape is kind of cool and then it will allow you to export the the corrected ortho photos so i want to grab a couple of photos here and i check this out ahead of time so we want photo 528 and 536 so i'm down in the photo tray and just held the control key down to select both of these photos here and these are of the sort of same area it's this sort of uh reddish brown vegetation sort of patched to this sort of region of the image and what i want to do is compare the original photos to the ortho photos for these so so i've got these guys here and then over in the workspace i'm going to right click and do export ortho photos okay gives me the pretty much similar options to just exporting the orthomosaic but with a couple of differences first thing to point out here is that it's giving me a template for the file name so it's going to go ahead and auto-generate those right i can just leave it like it is right now and then the other thing here at the bottom is whether we want to export all the ortho photos or just the selected ones and so we'll pick make sure selected is is uh is checked here and then we'll hit export and it's going to say where do you want these all right so in my lab 7 directory i'm going to create a folder called uh orthophotos okay and uh and put them in there and i'll take just a second here to export those ortho photos okay good so now what we want to do is switch over to a program like powerpoint or something like that and uh and see how these uh original photos compare versus the uh the ortho photos so all right so here's powerpoint and i'm gonna grab the original photos themselves and bring those in as images and uh they're kind of big to work with so i'm just going to go ahead and resize these to say like four inches high um yeah that should be pretty good okay and now i want to try to match these things up as best i can so i'm noticing here that um i'm going to need to rotate one of these uh relative to the other so if i go 180 degrees probably a little more than 180 degrees and then i'm going to try to match up this uh brown reddish brown vegetation patch here and let's get rid of that make this a little bigger so uh you can use these tools uh for sending things you know sort of backwards and forwards to try to match things up okay and that's not too bad there's only so good you're going to be able to do here with this because of the perspective displacement and the terrain displacement okay so you want to just try to get it as good as you can and uh and just kind of note the differences okay and then once you have it like as good as you think you can get it then uh take a take a screen shot of this or save this as an image that so you can put it in your in your lab document okay so uh let's go ahead and uh i guess i'll just use this blank slide up here and let's bring in our ortho photos there we go our ortho photos so um notice that the ortho photos of course like they're not square anymore right so because we have uh orthorectified those to account for the terrain and the perspective displacement um and with these also notice that i don't have to rotate um the one image they are already pretty well aligned okay i do have to sort of re scale them a little bit and the reason i have to do that is actually a powerpoint thing it doesn't have anything to do with the ortho photos themselves powerpoint is trying to kind of maximize the space that each one of these photos sort of takes up and uh so they're not scaled sort of correctly when i bring them in but if i monkey with this enough then it should get to a spot where they overlay pretty good which no this is not too bad all right so sitting back and forth so so notice that my my uh images sort of overlay just a lot nicer because we've corrected for the displacement uh in the image okay notice also i've got this sort of weird ragged edge at the end where it was uh you know getting a little uh funky with the that sort of high resolution surface model and uh and matching the uh the the seam lines figuring out where to pull from each of the images to construct it but from this sort of larger field area here things are looking pretty good so same thing here go ahead and take a screenshot or save this as an image and then you'll need that in your lab report okay so last thing let's flip back over here to to metashape and just make sure you save your document at the end when you're done and that is pretty much it for lab seven in the next lab we're going to shift gears a little bit and run this photogrammetry process with some multi-spectral data and uh and take a look at this parker farm area in uh in sort of this multi-spectral space so that should be a pretty good pretty good lab for us and we'll see you next time

Channel: Jason Karl

Views: 276

Keywords: drone, uas, remote sensing, photogrammetry, structure from motion, sfm, agisoft, metashape, orthophoto, orthomosaic, aerial mapping, drone mapping

Id: ECTdSgNdguU

Length: 18min 44sec (1124 seconds)

Published: Wed Oct 21 2020