There is one major problem in using open source tools such as QGIS and MapWindow for GIS work in Michigan. This is the apparent inability of the open source vector/raster map projection libraries to correctly define map projections of the class Hotine Oblique Mercator
(aka Rectified Skew Orthomorphic [RSO] in other applications). Most open source desktop GIS packages (like QGIS) incorporate these open source projection libraries (GDAL tools), and thus inherit this problem.
This is proving to be a serious impediment to the adoption of most open source desktop GIS software in areas where the Hotine/RSO projections are in use. Unfortunately, in Michigan, the Geographic Data Library of the State of Michigan makes GIS data (both vector shapefiles and raster image files) available for public download in the Michigan GeoRef projection. Michigan GeoRef is an instance of the Hotine Oblique Mercator projection. There are excellent, fully justifiable, technical reasons for using and distributing the data in Michigan GeoRef projection, but it causes monumental problems with many open source desktop GIS products.
Products from commercial GIS vendors appear to handle Michigan GeoRef with no apparent problems. However, when opening GeoRef-projected GIS data files using some open source desktop products like QGIS, the results are unpredictable: the North arrow in the map window may flip horizontally, GIS layers in other projections may not plot correctly (whether or not the "project-on-the-fly" option is selected), and map scales can be grossly inaccurate. We need to convert the GeoRef-projected files to the State Plane Coordinate System (or some other projection) before they can be used in QGIS.
To work around this problem, this project has adopted the procedure of using another open source desktop GIS product, uDig, to perform this conversion. The uDig software appears to read GeoRef-projected shapefiles correctly, and successfully converts them to other projections.
View the procedure as a slideshow: CLICK HERE
or on the figure below right. (When the slide show appears, press the "pause" button [two vertical bars] between the left and right arrows. This will stop the overly fast automatic display of the slides. Use the arrow keys to view the slides one at a time. )
The step-by-step procedure is as follows:
Using uDig and QGIS to Convert a Shapefile from MI GeoRef Projection to State Plane MI Central
- Fire up your copy of uDig.
In the "project" of your choice in uDig, create a clean (new) map. (Right-click on your project in the "Projects" window on the upper left of your uDig map screen, then click on "New Map.") Make sure your uDig map window is on the new map you've just created.
- Add the shapefile you wish to convert from Michigan GeoRef project to State Plane. (Layer > Add > Files > Next , then navigate to the location of your GeoRef shapefile. Select your shapefile, then click on the "Open" button. Your GeoRef-projected shapefile should show up in the uDig map window.
- Look in the "Layers" window just below the "Projects" window. The layer you just brought in should be listed with a check in the check box.
- Right-click on the layer name (adjacent to the check box). You will get a context menu, in which one of the choices is "Export ..." . Click on that "Export ..." choice.
- You will get an "Export" dialog box. Click on "Layer to Shapefile", then click on the "Next >" button.
- You will find yourself in the "Export to Shapefile" dialog box. This is the tricky part !!!
- Check the "destination folder" box at top. This is where your converted shapefile will be placed. If you're OK with the location that's there, then go to the next bullet. If the location there is not what you want, "Browse" to the folder you want the converted shapefile to reside, then press "OK."
- In the "Resource List" box, you will see a check box (checked) and three items you need to edit: the "destination folder," the name of the converted shapefile, and the map projection to which you want the new shapefile converted. What is shown at this point is the original name of the GeoRef-projected file and the GeoRef projection. So here's what you do:
- In the destination folder box, if what's there is not what you want, browse to the folder where you want your converted files to be saved, the press "OK"
- Click on the name of the file. The name will turn blue, and you can edit the name. Change the name to something else. (I usually put something in the name that shows its projection.)
- Click on the projection name. If it's a file from Michigan CGI, you should see something like, "NAD_1983_Hotine_Oblique_Mercator_Azimuth_Natural_Origin". Click on the button ("...") to the left of the name that will give you (after a brief pause) a list of the map projections you can choose. Scroll all the way down to "NAD83 / Michigan Central (ft) (2252)" and select it. Make sure you've selected the one that ends with "(2252)" !! Push "OK" to return to the "Export to Shapefile" menu. Check and make sure that the Destination Folder, File Name, and the desired Map Projection are correct. Press "Finish" on that dialog.
- You'll see a bar that indicates progress in the conversion process. When the bar and dialog disappear, the conversion process is finished, and you should see a new shapefile projected to the State Plane coordinate system in the folder you specified.
- We'll check to see that our conversion worked. Fire up your copy of QGIS.
- From the blank QGIS screen, go to the "Settings" menu and click "Project Properties ..." We will be setting the map to the projection of our choice.
- In the "Project Properties" dialogue window, click on the "Coordinate Reference System (CSR)" tab to bring up a list of map projections.
- Scroll to the "Projected Coordinate Systems" Item. That's the second major item, right after "Geographic Coordinate Systems." Click on the plus sign if it's there next to the item to expand the section. You'll see a whole bunch of projected systems below the "Project Coordinate Systems" major heading. Scroll down the list and find the "Lambert Conformal Conic" heading. Our MI State Plane Coordinate System is one of the Lambert Conformal Conic class of map projections. If there's a plus sign next to the "Lambert..." heading, click it to expand the section.
- Once you've expanded the "Lambert Conformal Conic" item, scroll down in the projections listed thereunder until you find "NAD83 / Michigan Central (ft) --- EPSG:2252" and select it. Then go down to the "OK" button and click it.
- This brings us back to the blank QGIS screen. The QGIS project is now set for the MI State Plane Coordinate System, MI Central. Check to see that the CRS window at the lower right shows EPSG:2252, which is the international code (maintained by the European Petroleum Survey Group or EPSG) for the MI State Plane, Central Zone. (The US Federal code for the same is FIPS2112)
- Now bring in the newly-converted shapefile. It should plot with no problems: check the scale bar and the north arrow to ascertain that they are correct.