NZ Rail Maps: Using Gimp To Georeference Retrolens Aerial Photos [6]: Alternate Overlaying Technique

Today I am going to look at an alternative way of stitching together the source layers. I have to do a set of mosaics for the station of Kamo, which is the northernmost part of Whangarei. From a rail perspective there were two stations there. Just a little south of Kamo there was a coal mine which appears to have had a siding going off on the west side of the line, and although I don’t know any more than that, I will include the NZR aerial images from Kamo right down to this area, since they are for continuous corridor. The idea that I am working on today is to reduce the amount of overlap between aerial layers. With every other project to date I skipped every second layer in the sequence but still found there was a lot of overlap between layers, enough to create various challenges I mentioned with overlapping, but not enough to be able to skip to every third layer. I want to look at reducing those overlaps, the potential benefits being a lot faster to overlay them in sequence, smaller file sizes and more accurate representation of the ground below with reduced perspective distortion. The way I did it before with every second layer was still overlapping quite a percentage of each layer and it being overlapped at one side, which means one side of the image will be more distorted.
In the layer sequence for Kamo down to the colliery siding the images are Survey 2788, Run A 1 to 3 and 2788 B 1 to 8. A total of 11 source layers. But I am going to use only the middle of each one of those. In addition because there is overlap between the A run and the B run, as is fairly common when the same rail corridor is covered by more than one run, because it goes around a curve and goes out of the side of the straight line of the existing run, I might be able to drop one or two layers. So what we need to do is to run Gimp with no projects open and open the source layers up using the Open as Layer command. Gimp creates a new canvas the size of the first layer and loads all the layers into it, centering them within that canvas. As all the source images are more or less the same size, they all line up with each other more or less exactly, or at least good enough for this exercise.
The obvious approach is to select a percentage of all of the source layers and apply that percentage to all of them. Using the centremost part of each layer is a good rule of thumb but you will have to check if this works with the source images you are using. But because you are using all of them, there is so much overlap everywhere that it’s highly likely it will. The way to do this with Gimp is to use the Guides that you can set up from the Image menu. First of all, go to the View menu and select Show Guides. Then go to the Image menu and come down to the Guides submenu and if there are any existing guides visible and you don’t know what they are then Remove All Guides. Then on the same submenu, New Guide (by percent). In the dialog select vertical or horizontal (you need guides that are at right angles to the rail corridor, but I set up both orientations to cater for both possibilities). Start with two guides at 33.33% and 66.66%.  The next step is to go through the layers one at a time and look at where the guides fall on them for example from A 1 to A 2 and see if there is enough overlap. In my case I decided to go for the middle 40% and therefore I went back, removed the guides and set them to 30% and 70% then there was enough overlap.
The next stage is to crop the layers and save them to be imported into the mosaic project. First thing to do is to create a selection that covers that middle 40%, which I do firstly by going to the View menu and turning on Snap to Guides, then I create a rectangle selection and use the guides to get the long edges into the right place. The short edges go up to the border, which we need to trim off in any case. This is why I didn’t mention cropping the frame off each source layer as I did when I imported the full size layers directly into Gimp as I mentioned in a previous post. The way I am going to draw this selection is that it is a rectangle where the short edges parallel the rail corridor and the long edges are at right angles. This is how I get only the middle 40% of the rail corridor on each layer.
There are a couple of techniques for cropping in Gimp, just as a by-and-by, with different results in terms of their impact on the source layers. The obvious one to use is the Crop to Selection option on the Image menu. This will take the canvas down to the size of the selection and at the same time it will crop every layer to fit that canvas. As it happens this is the right thing to do with this project. The alternative which is useful to know if you want to keep the source layers at their full size, is to choose the option on the Image menu to Fit Canvas to Selection. That doesn’t have an application to this particular project at all, but there are times when it’s useful to have the full size layer available even if part of it is outside the visible canvas area and I just mention that in passing in case one day that is the situation you have, and there have been times I have used this alternative, for example when selecting a few layers from off a bigger canvas and saving them in a small canvas. Gimp will save layers that are outside the visible canvas, obviously using up more disk space than if all the layers are cropped within the canvas. Keeping the canvas just to the size you need is important because I discovered a bug in Gimp with a large canvas with only one small corner occupied that it will crash when saving. So my rule of thumb is only create the canvas size you will use and expand it as needed.
So use this Crop Image to Selection option to crop down all the layers at once. Then save to a new Gimp project file. I have 11 layers (A 1 to 3 and B 1 to 8), and the new project file has saved all of the middle 40% of 11 layers at less than 1 GB total, which looks really good so far. Now I am going to import additional base layers and my 11 segments of overlays into my mosaic project. In this case my base layers go from 93Y35-92S5Z to 93Y3B-92S63, which is a total of six columns (Y35-36-37-38-39-3B) and five rows (S5Z-60-61-62-63), so the first task is to increase canvas size by adding 36000 pixels to the top, the canvas being already wide enough for the six columns. My new canvas will therefore be 151200 x 52800 pixels since I added an extra blank row to ensure a one-row gap in the canvas between this geographical area and the one appearing on the canvas below, as I have multiple areas (different railway stations) shown on the same canvas. One thing when I started doing large projects is it doesn’t matter how much of the canvas is actually used. What uses the disk space is each layer, regardless of where it sits in the canvas. I have had some very large canvases that only have an L shaped area along two sides occupied, maybe only 15-20% of the entire canvas, and there are no issues like wasting disk space because Gimp only saves the actual layers, not the canvas itself. It just saves the measurements of the canvas and the position of each layer, not every single pixel of the canvas itself. 
After resizing the canvas, use Open as Layers to bring in the base layers, being sure to first select in the Layer list the layer that you want the new layers to be inserted above, and if you want the new layers to appear in the right order then in the file dialog box select a sort order that is the reverse of the one you want. Then once the base layers have been imported and positioned in the right places in the canvas, bring in the overlays. You do this using the Open as Layers command again, but instead of selecting image files to bring in as one layer each, you select your project file that you created from cropping the overlays.  Gimp brings in all the layers from that project file into your current project individually, inserting them above the current position in the layer list in the same order as you had them in their project file.
Then you can overlay them the same as usual. That turned out for me to be a lot easier working with smaller layers, and the result was they came together in the mosaic project quite rapidly. I did change the layer order in one place, putting layer A 3 above A 2, and was able to dispense with layer B  1 because B 2 had enough overlap onto A 3, and I also found I didn’t need layer B 8, so that got me down to 9 layers out of the 11 I thought I would originally need. After putting in the first layer, I can speed up the subsequent layers by just dragging them into the same size as each previous one and then do the underlap straight away, then line up the bottom edge. Even with the potential challenges of surrounding hilly terrain not lining up and significant changes in the general area – there is now a lot of residential development on the east side of the railway at the first Kamo station it was very straightforward. (Kamo had two stations both of them named Kamo, the earlier one being further south than the later one, and as both were open at the same time, the more northern one was originally named Ruatangata or Ruatangata Springs.) The amount of overlap (and therefore underlap) is much less and the narrower layer segments, for various reasons, are simply a lot easier to work with – because they are a closer representation of the actual terrain. I believe the result has fewer overlapping errors and is a more accurate representation of what is on the ground overall but this will have to be tested out in practice, and that is much more likely to be tested in mosaics covering large rail yards than with a station with a few tracks or a single track main. It’s hard to work out any possible improvement in file size but as an exercise I went back to my original project, removed every second layer and the redundant layers and then saved it, the result was a file 50% larger than the original. When I saved my mosaic project in Gimp, it already had the full size versions of all the layers in Run A and Run B imported in, so it’s hard to pick up the actual saving in disk space that might have resulted. I’ve taken out 17 full size layers and replaced them with 9 that are less than half the original size, but in their place I imported another 16 base layers. But anyway the new version of the mosaic project file is about half a gigabyte smaller than the previous version. So the whole I think there are going to be some physical benefits in resource usage as well.
So the net result of this exercise is that is how I will be doing my mosaics in future. Taking the middle 40% slice out of consecutive layers because it works out better all round. The overlaying process at this point can be described as:
  1. Align and size an overlay along the long edges of the previous overlay (does not apply to first overlay)
  2. Find the underlap point with the previous overlay (overlap of background for first overlay) and roughly align to it.
  3. Line up the bottom edge to roughly the right place
  4. Do Step 2 again more precisely
  5. Do Step 3 again more precisely
  6. Transform.
There is one more part to write and that is how to extract the mosaic tiles for the GIS. That will be Part 7 next time.