Chris -
Well, now you've gone and made me feel I have to add a little detail that I thought we could just sweep under the rug for now <g>. If your aerial photos are indeed in the UTM projection as we suspect, and you georeference them in your world file as if they were latitude/longitude, it MIGHT seem to work over a small area. But it will be subtly incorrect. If your image is really in UTM, and you look at the corner coordinates in latitude/longitude you will find that the latitude of the top left corner is NOT the same as the latitude of the top right corner. In other words, the correct X coordinate of a pixel is a function of both X and Y, and that's not a concept you can express in your world file. But if the area is small the difference (error) will also be small, and it will look, at first glance, as if it works. But you will see a positioning error that will vary in magnitude depending on your position on the image, and the error drift in your markers sounds very much like you're seeing the problem I'm talking about. If that's the case, then correcting the projection and world file to accurately describe the image as being in UTM should correct the problem. You will also see the same symptom, of course, if the problem is reversed and the image is in lat/lon but you're treating it like UTM. The best way to check is to calculate both the UTM and lat/lon coordinates of all four corners of your image. It's supposed to be a rectangle, so the top left Y value should match the top right Y value, the top right X value should match the bottom right X value, etc. You should find that that holds true (or close to it) for only one of your two coordinate systems - that's the right one. - Ed Ed McNierney TopoZone.com ________________________________ From: UMN MapServer Users List [mailto:[EMAIL PROTECTED] On Behalf Of Christopher Harris Sent: Thursday, September 27, 2007 12:24 PM To: MAPSERVER-USERS@LISTS.UMN.EDU Subject: Re: [UMN_MAPSERVER-USERS] Rasters, TileIndex and Shapefiles - Oh My! Really Confused Great! Ok. Well, it looks like you've armed me with enough info to take over the world - at least on my computer screen. You've cleared up a lot of mysteries that I've had thus far. I'm going to go apply all this and see what explodes and doesn't explode. I do have one problem I can think of with the app that I already had created, but I'll wait to see if it goes away after I apply all of this helpful info. I don't think it will. I'm pretty sure it involves how I'm setting up my extents (Markers are always a few hundred yards off and each marker that is located further north rotates north and east following a circular counter-clockwise pattern). But, don't worry about that now. I'll try and figure it out. Thanks, Ed. You are a very patient man and very nice too. Sorry it took a while for me to see the light, but I usually learn best when seeing examples, diagrams, pictures, etc. So, typically I'll have to draw or write stuff out to get the idea. I'm more of a visual person when it comes to memory. - Chris ________________________________ Date: Thu, 27 Sep 2007 15:03:47 -0400 From: [EMAIL PROTECTED] Subject: Re: [UMN_MAPSERVER-USERS] Rasters, TileIndex and Shapefiles - Oh My! Really Confused To: MAPSERVER-USERS@LISTS.UMN.EDU Chris - In my previous message (quoted below), I said: #3 "The X pixel dimension will be (maxx - minx) / xpixels, and the Y will be (miny - maxy) / ypixels. You should find that the X number is -1 times the Y number." And that is correct. Maxx IS the "greatest number in lat/lon or meters", etc. Maxx - minx will be a positive number, and miny - maxy (note that they're reversed) will be a negative number. - Ed Ed McNierney TopoZone.com ________________________________ From: UMN MapServer Users List [mailto:[EMAIL PROTECTED] On Behalf Of Christopher Harris Sent: Thursday, September 27, 2007 11:49 AM To: MAPSERVER-USERS@LISTS.UMN.EDU Subject: Re: [UMN_MAPSERVER-USERS] Rasters, TileIndex and Shapefiles - Oh My! Really Confused Ok.....sorry it's taken me a while to post again. Got caught up with something else. Considering the info and dimensions I have right now pending on what you say about how I got those tiled images. In Lat/Lon: TL - lat = 40.117268 lon = -88.248281 BR - lat = 40.082274 lon = -88.205980 In Pixels: Image Width = 2602 Image Height = 2800 X pixel dimension: (-88.205980)-(-88.248281) 0.042301 ________________________ = _____________ = 0.000016257 2602 2602 Y pixel dimension: 40.082277-40.117268 -0.034991 ________________________ = _____________ = -0.000012497 2800 2800 Yeah, I see how I reversed them. I was thinking max was the greatest number in lat/lon or meters and not in regard to the image and its coordinate system. With the numbers provided so far, his should be right - right? - Chris ________________________________ Subject: RE: [UMN_MAPSERVER-USERS] Rasters, TileIndex and Shapefiles - Oh My! Really Confused Date: Wed, 26 Sep 2007 18:25:52 -0400 From: [EMAIL PROTECTED] To: [EMAIL PROTECTED]; MAPSERVER-USERS@LISTS.UMN.EDU Chris - (I added the list back to the reply line - please always reply to the whole list - thanks) A world file is in the units of the image's projection, not the other way around. Having a world file does not mean the image is in UTM projection. The UTM projection is a popular one, but it's certainly not the only one. For a map in Illinois UTM is certainly a possibility, but an Illinois State Plane Coordinate System projection is also a possibility - especially if the original imagery came from the State of Illinois. And geographic lat/lon coordinates are also a possibility due to the increasing number of people in the world who seem to think the Earth is flat (at least on the Web). I don't know which campus you're using, but if you're at UIUC then the UTM coordinates will be in UTM Zone 16 with values of around 395000 (X or Easting) 4440000 (Y or Northing). If your numbers don't look like those, they're not UTM. Oh, sorry - I started answering before reading all the questions, and you later seem to confirm that your coordinates are indeed UTM. But you got the formulas backwards, and X should be positive while Y should be negative- not the other way around. And the measurements in your world file are indeed in meters, but that's because those are the native units of the UTM projection being used. There's nothing that requires them to be in meters; many US state plane coordinate systems use units of feet, and those would appear in the world file, too. Now it is highly unlikely that your pixel resolution is different in each dimension. You could average those two numbers, or double-check your coordinate values (you didn't say what source you used to get them). If you got this image from a GIS source, I would say that if you think the dimensions are different you're wrong, but who knows what the University did to munge the image into a pretty picture. So maybe the dimensions ARE different; that's not impossible, just odd. I still would double-check your measurements and math, but if you get the same answer that's OK. Once you do that you just have to crank out those world files! - Ed Ed McNierney TopoZone.com
