Reproject Michigan Georef images from Oblique Mercator to UTM

Reproject Michigan Georef images from Oblique Mercator to UTM

Before we get started, let me attempt to persuade not to do this. There are already available UTM projected images of most if not all, of the generally available Michigan Georef images. See: http://datagateway.nrcs.usda.gov/. If for some reason, you simply must do it the hard way, or you have Michigan Georef images that are not available in UTM projection, then by all means continue.

This task can be accomplished entirely using freeware tools! Here's a list of tools that you'll need:

Tiff utilites Geotifcp and Listgeo. These are not the most current versions, but they're the easiest to install and they'll suffice for our uses. Unzip and install them.

Erdas ViewFinder. Install ViewFinder.

You may also optionally need an image editor program that can create packbits compressed tiff files. IrfanView is a poplular freeware program that will do the task. If you prefer to create images soley for use with OziExplorer, then you can use the Ozi utility; Image to Ozf instead.

It may be helpful also to have the geotiff add-on for Ozi installed. Unzip it, copying the file to the OziExplorer folder

Now that you've got all of the tools needed, let's get started.

1. Download the offending image. That is, get a DRG from the Michigan DNR website. Once there, click on the "Geographic Extent" link and then the "County" link. Select the county for which you wish to download the DRG mosaic, by clicking on it. For this tutorial, we'll be using the Jackson county DRG. Next, scroll way down towards the bottom, until you see the section titled "TOPOGRAPHY". The one we're looking for is the 24k Digital Raster Graphic (DRG). These files run anywhere from 20 to 50 megabytes. So, a high speed internet connection is a must. If you're having trouble following the links, here's a direct link to the file that we'll be working with: http://www.dnr.state.mi.us/spatialdatalibrary/sdl2/topography/drg/Jackson_DRG24k.exe. This is a self-extracting zip file, so go ahead and run it, copying the files to a convenient folder of your choice.

2. Open a command prompt and navigate to the folder to which you previously copied the DRG files. Make sure that you've previously copied the geotiff utilities to this folder. On the command line, type:

listgeo jackson_drg24k.tif >jackson_drg24k.ref

Open the jackson_drg24k.ref file in any old ordinary text editor. The standard "Edit" will do. It should look something like this:

Corner Coordinates:
Upper Left    ( 605325.780, 209968.695)
Lower Left    ( 605325.780, 169740.483)
Upper Right   ( 654458.815, 209968.695)
Lower Right   ( 654458.815, 169740.483)
Center        ( 629892.298, 189854.589)

Now, download the the following file:

http://home.pacbell.net/lgalvin/mom.geo

saving it to the same folder with the DRG. Open it up with a text editor, along with the .ref file from above. What we need to do here, is to transplant the first 10 lines from the .ref file, to the .geo file. It's important that precisely these 10 lines, are the only ones transplanted. These lines contain the reference information for the specific image with which we're working. The .geo file is a generic template that contains a complete description of the Oblique Mercator projection used for these images. By transferring the first 10 lines, we're creating a complete GeoTiff description of the Jackson county DRG image. When you're done, it should look EXACTLY like this:

Geotiff_Information:
   Version: 1
   Key_Revision: 1.0
   Tagged_Information:
      ModelTiepointTag (2,3):
         0                0                0
         605325.78        209968.695       0
      ModelPixelScaleTag (1,3):
         2.46466193       2.46466193       0
      End_Of_Tags.
   Keyed_Information:
      GTModelTypeGeoKey (Short,1): ModelTypeProjected
      GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
      GeographicTypeGeoKey (Short,1): GCS_NAD83
      GeogAzimuthUnitsGeoKey (Short,1): Angular_Degree
      ProjectedCSTypeGeoKey (Short,1): User-Defined
      PCSCitationGeoKey (Ascii,39): "Projection = Oblique Mercator (Hotine)"
      ProjectionGeoKey (Short,1): User-Defined
      ProjCoordTransGeoKey (Short,1): CT_ObliqueMercator
      ProjLinearUnitsGeoKey (Short,1): Linear_Meter
      ProjCenterLongGeoKey (Double,1): -86
      ProjCenterLatGeoKey (Double,1): 45.3091667
      ProjCenterEastingGeoKey (Double,1): 2546731.5
      ProjCenterNorthingGeoKey (Double,1): -4354009.82
      ProjScaleAtCenterGeoKey (Double,1): 0.9996
      ProjAzimuthAngleGeoKey (Double,1): 337.25556
      End_Of_Keys.
   End_Of_Geotiff.

Save the edited file as: jackson_drg24k.geo. Exit the text editor and return to the command prompt.

3. Type:

geotifcp -g jackson_drg24k.geo jackson_drg24k.tif jackson_drg24k_geo.tif

What this does, is to read in the DRG and insert the GeoTiff description that we created, into a newly created DRG, jackson_drg24k_geo.tif. I know that this seems like a lot of rigamarole, but it's necessary to have the DRG completely described, in order to do the reprojection.

4. Download the following file tiff.pdf by right-clicking the link and choosing "save as". If you try to open it directly, Adobe Acrobat Reader will attempt to open the file and since it's not actually an Adobe file, it will fail. So, save this file to your Program Files\Erdas\ViewFinder\defaults folder, overwriting the one that's there. This changes some settings in ViewFinder, that make the tiff images it creates, more compatible with other programs.

5. Start Erdas ViewFinder. Navigate to and open the jackson_drg24k_geo.tif file that we had previously created. Click on File and choose Save As. From the "Files of Type" dropdown box, choose Tiff. Navigate to the folder to which you wish to save the re-projected image file. Type "jackson_drg24k_UTM.tif" for the filename. Next, click on the Output File Options tab at the top of the dialog box. Put a check in the box "Change Output Projection" and remove the checks from all the other checkboxes. In the dropdown box "Category", choose "UTM Clarke 1866 NAD27 North", if you wish the final image to be using NAD27 datum, or "UTM WGS84 North", if you wish the final image to be using WGS84 datum. In the "Projection" dropdown box, choose UTM Zone 17. Click the OK button once, to register your selections and again to start the reprojection process.

6. Now, go away for an hour or so. Not that it takes that long, but you should take a break by this time. After returning, close Erdas ViewFinder and you now have a UTM projected DRG of Jackson county, that will import directly into OziExplorer and just about any other GIS or map viewing program. So, go ahead and try it out. Now, you may notice that the Tiff file that ViewFinder created is quite a bit larger than the original image file. That's because ViewFinder has created an uncompressed Tiff image. We can fix that in the next step.

7. Optional file size reduction. There are two avenues to pursue here, but before we get started, make sure that you've already imported the image into OziExplorer and saved a .map file for it. If you want to create a generally portable image file, then you need to open the Tiff image in IrfanView and re-save it as a Tiff image, but this time with the option of packbits compression. You can make it even smaller, if you choose LZW compression, but this is not compatible with many Tiff file viewing programs. OziExplorer can be made to read LZW compressed Tiff files, but it's not the default. So, it's best to stick with packbits compression. One unfortunate side-effect of using a general purpose image editor like IrfanView, is that it's totally unaware of the GeoTiff tags and keys that are embedded within the Tiff image file. So, these will be lost, when you re-save the file. For the really advanced reader, you should be able to figure out how to re-insert them. It won't generally be a problem, not having the GeoTiff tags and keys embedded within the file, unless you haven't already imported the image into OziExplorer before resaving it in IrfanView.

If you're creating an image exclusively for use with OziExplorer, then you can use the Ozi utility "Image To Ozf", instead of IrfanView, to greatly reduce the file size. Startup the Image To Ozf program and in the "Source Image Folder" dropdown, navigate to the folder where the image is stored. Similarly, select the "Destination Folder" in which you wish the final image created. Make sure that you have the "Map File Path" set to point to the location of the .map file for this image. Put a check next to the image file you wish to convert. Uncheck the "Initial Resize" and "Number Colors" checkboxes. Click on the button "Process Image Files to OZF2 Files". After a while, your final ozf2 image file has been created. In OziExplorer, you can change the .map file to point to the new .ozf2 file, instead of the old .tiff file. At this point, if you're absolutely certain that you'll never need this image for any other program than OziExplorer, you can delete the all of the .tiff files that we've worked on.

As always, direct questions to:

l_g_a_l_v_i_n_@_p_a_c_b_e_l_l_._n_e_t

or alternatively, to http://groups.yahoo.com/group/OziUsers-L/