• Importing And Using Orthophotos In WED

    Importing And Using Orthophotos (non-GeoTiff) in WED

    By Richard Elliott

    WED is very much geared to US designers who have access to the USGS orthoimagery with embedded coordinate data. For the rest of the world, importing images into WED is much more complicated and puts a lot of people off. I hope these notes will help a bit, but be warned! It isn't easy, some of the software tools you have to use are not very user friendly and for some things there is no alternative to using pen, paper and a bit of maths to achieve what you want.

    There are two reasons you might want to import an orthophoto. The first is simply to give you a background on which to position things in WED which, unlike Overlay Editor, doesn't let you see the default X-Plane terrain layout. If that is all you want, and the orthophoto is not going to appear in the finished scenery, it isn't too difficult. If you want to create orthophoto polygons, which will appear in the scenery, it is a lot harder. Either way, the first thing you need to know is how to find, download and import an orthophoto, so let's start with that. I will assume you are familiar with the basics of using image processing software like GIMP or Photoshop.

    Sources Of Orthoimages And Downloading Tools

    If you want free images you only have a choice of Google Maps or Bing Maps. The images are free to download for use in your own personal scenery. If you redistribute them you are in violation of copyright. In fact Google or Microsoft are very unlikely to take any action against a freeware developer but sites like xplane.org are likely to reject your work. You can get hold of licensed orthoimagery from sources like the Ordnance Survey or private companies like Getmapping, but it costs an arm and a leg.

    Many people just take screen shots in Google or Bing and try to position them in WED. That is not a good way to go because you don't have any accurate control over the resolution of the images or the exact geographical coordinates of the corners. It all depends on how much you zoomed your screen view so it becomes mostly guesswork. The better solution is to download the raw image data. There are two main freeware tools for doing this, each with its pros and cons.

    Noni Map View is the easier one for beginners. It can download images from either Google or Bing (or OSM, but those are just maps) and has a nice graphical user interface. You have an onscreen image from the selected source and you zoom in on the area of interest, just like you would in Bing or Google Maps themselves. You can set the resolution of the screen image (the zoom factor) and the resolution you want to download separately, so you have total control over the resolution. You can use the screen to preview what the image looks like at various resolutions, choose the one you want to download and then zoom the screen out again to make the next step easier. Then you select the area you want to download, either with the onscreen "select" tool or by typing in the coordinates in the boxes, and start the download. Bing downloads are much faster than Google. The actual map tiles on the server won't match the exact coordinates you selected but the software sorts that out. It downloads a set of map tiles covering the area you selected, trims them as necessary and stitches them together into a jpeg image you can import into WED. The exact coordinates of the corners are in the Noni Map View screen and if you hit "save project" you can retrieve it for later use.

    That sounds quite painless, and it is if you just want an overlay image for positioning purposes. The downside is that if you want to make orthophoto polygons you have to cut up your image into squares with side lengths (in pixels) that are powers of 2 and you have to know the corner coordinates of each square. The size, in pixels, of what you have downloaded depends entirely on how big an area you selected, and cutting it up into suitable tiles will involve a fair bit of geometrical puzzle solving.

    Google Satellite Maps Downloader is the other tool. You may find the next couple of paragraphs very discouraging but read on. This tool actually has a huge advantage if you want to make orthophoto polygons.

    It only works with Google Maps and the interface is a fairly primitive non-graphical text box. There is no map preview, so before using it you have to go into Google Earth and zoom in on your area of interest. As you move the screen pointer around in Google Earth you will see the coordinate read-out changing at the bottom of the screen, so you move it around to work out the top and bottom latitude and right and left longitude of the image you want to download and note the figures down on a piece of paper. Then close Google Earth, open GSMD, start a project and enter the coordinates in the boxes. You also have to enter a resolution factor and you have no way of previewing what it looks like or what size the final image will be. That just comes with experience. The number of tiles downloaded also depends on the resolution and you have no way of predicting that either. Also the free version only lets you download up to resolution 15. You have to pay to register the product if you want more.

    Then you download the tiles. It downloads a stack of raw 256x256 pixel image tiles covering the area you specified. When it is finished you can open a tool which stitches those tiles together into a bitmap file covering the whole area. You can use that bitmap as an overlay image in WED, but first you need the coordinates. They are not quite the ones you asked for. Because this tool just downloads raw tiles, with no trimming, it will have adjusted your coordinates to match the nearest tile boundary available on the Google server. To find out the actual coordinates, you open the log.txt file it created in the project folder. In the first few lines you will find the coordinates you requested and the actual ones it downloaded. You can use those to position the image in WED.

    It sounds like a lot more bother than Noni Map View, and so it is if you just want to make an overlay. But look again at the tile size. You have a stack of square tiles with a size that is a power of 2 and the corner coordinates of every tile is in the log.txt. They are therefore all ready to be converted directly into orthophoto polygons. The 256 pixel size is a bit small to be useful but of course you can easily paste them together in groups of 4 to make 512x512 tiles, 16 to make 1024x1024 and so forth. No maths, no fiddly image processing, just a simple paste job. For many people that outweighs all the earlier disadvantages.

    A final downside, though, is that Google map tiles have Google watermarks on them at multiple levels. The closer you look the more you find. Most of them are quite unobtrusive but you would probably want to do a bit of touching up with your image software to remove the more obvious ones. Bing downloads don't have these.

    Tags: orthophotos

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