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Obtaining Color Aerials for your QGIS Project

One of the best sources of aerial photographs for use as basemaps for GIS products is your local county aerial photographs. Usually they are high resolution (typically less than 1 m) black-and-white or color imagery, already georeferenced and ready for use in your desktop GIS software.  These are typically flown in late autumn or early winter when there are no leaves on deciduous trees. Equally typically, unless a Local Unit of Government (LUG) has partnered with the county (i.e., contributed to the cost of flying and developing the aerials), a LUG may have to pay the county for access to these orthophoto files. An example of county aerial imagery is the Grand Traverse County high-resolution 2010 orthophoto.  The image below shows Acme Township, the north side of the intersection of M72 and Bates Rd.  It is shown here as it was rendered on QGIS (native projection is MI State Plane Coordinate System):

Very recently, an excellent, reasonably high-resolution, set of aerial imagery from the National Agricultural Imagery Program (NAIP) of  the US Department of Agriculture (USDA) has become available for use by the general public, free of charge. It is served over the internet by one of the WMS map servers of the US Geological Survey (USGS).  This imagery is at 1-m resolution. Unfortunately, because the one of the purposes of the photography is to survey US crop coverage, the imagery was created during the growing season (leaf-on).  However, due to its high resolution, under most circumstances it can serve as an adequate replacement for county aerial orthoimagery.

The NAIP imagery from the USGS server natively supports a number of map projections, but the three zones of the Michigan State Plane Coordinate System are not among them.  Web maps using this imagery need to use the WGS 84 Geographic Coordinate System (EPSG:4326) as the datum and map projection.  However, for desktop GIS projects, the imagery can be projected to the Michigan State Plane Coordinate System in QGIS (and uDig, among others), and used as the base imagery onto which GIS layers can be overlaid. A sample of the same area as above, at the same scale, is shown below using the NAIP imagery on QGIS, transformed to the NAD 83 datum and reprojected to the MI State Plane Coordinate System:

The rest of this section comprises a short tutorial on bringing the NAIP imagery into QGIS from the USGS WMS server.  The instructions are at the left.  Click on the corresponding figure on the right to view an enlarged version of each graphic.

1.  Fire up QGIS.  Go to the "Settings" menu and click on "Project Properties."  We will first set the project and map properties!  (click on figure to enlarge ->)

2.  In the "Project Properties" dialogue window, click on the "Coordinate Reference System (CSR)" tab to bring up a list of map projections.   Be sure the "Enable 'on the fly' CRS Reprojection" checkbox is CHECKED !  (click on figure to enlarge ->)  

3.  Scroll to the "Projected Coordinate Systems" Item, then click on the plus sign if it's there to expand the selection, and then go down and find the "Lambert Conformal Conic" sub-section. Our MI State Plane Coordinate System is one of the Lambert Conformal Conic class of map projections.  (click on figure to enlarge ->)

4.  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.  Make sure that the 'Enable "on the fly" CRS reprojection' checkbox is checked!

5.  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 for the MI State Plane, Central Zone. (The US Federal code for the same is FIPS2112)

6.  Before we bring in the NAIP orthoimagery from the USGS WMS server, it will be best if we bring into QGIS a vector layer that covers our area of interest. Our map bounds will be set by the layer (or by our subsequent zooming into that layer). This way, when we bring in the orthoimagery, it will already be zoomed in to our area of interest.

(If we bring in the orthoimagery first before any other layers, we'll initially get a distorted image of the entire United States, and we will have to zoom in to our area of interest. In the image of the US, the only undistorted area will be the band of area that defines Michigan Central in the State Plane Coordinate System.  Once we bring in the NAIP layer, zooming in and out of takes more time than normal. This is because a new map image is requested and downloaded from the USGS server every time we change zoom levels or map bounds, redrawing the background orthoimagery.)

In the figure at left, we bring in a vector layer from Acme Township.  It is the shapefile of discrete points that shows the July 2011 locations of stands of the invasive phragmites reeds along the Acme shoreline of the east arm of Grand Traverse Bay.

7.  Now go to the Layer menu, and click on Add WMS Layer ... menu item.  Or, you can press CTRL + SHIFT + W from the keyboard to go straight to the WMS layer dialogue.

8.  QGIS doesn't know about the NAIP orthoimagery, so we need to tell it where to go and find it.  We also need to make it remember that information.  In the Add Layer(s) from a Server menu, press the New button in the Layers tab.

9.  In the Create a new WMS connection dialogue, type in a useful name for the WMS connection in the Name box.  Then, in the URL box, type in the following “get capabilities” URL.  If I were you, I'd just copy the URL from below and paste it into the dialog box:

Be sure to include the question mark at the end. Copy and paste the above URL from this page into the URL box.

When done, press "OK."

10.  The data you entered for connecting to the USGS server has been saved, and is shown as a new entry in the list of servers you have saved earlier (if any).  

The connection URL is the WMS get capabilities request.  When QGIS connects to this URL, the USGS map server will send a catalog of  its available map layers that can be downloaded and rendered on QGIS.

Press the "Connect" button to get a list of the imagery available from the WMS server.

11.  The USGS catalog shows only one layer that's available, the 2010 NAIP orthoimagery.  Select the layer, then press "Add" to add the layer to your QGIS project. 

While the imagery is being added, press "Close" to get rid of the dialog.

12.  The NAIP imagery is downloaded.  Because it was loaded after the phragmites shapefile, it renders over, and covers, the shapefile.  Note that the QGIS brings in the imagery already zoomed in to cover the extent of the phragmites shapefile layer.  This is because you brought in the phragmites layer first, forcing QGIS to automatically zoom in the imagery to cover the same area as the extent of the phragmites layer.

(Review the writeup for step 6 on what would have happened if you had brought in the aerial imagery before you loaded the phragmites layer.)

13.  By reversing the order of the two layers in the layers list, the phragmites layer is now plotted over the orthoimagery.  The QGIS project is in NAD83 datum, Michigan State Plane Coordinate System projection (Michigan Central).

Now that the USGS web map server is stored in QGIS, you will be able to bring in the NAIP layer by invoking the Layer > Add WMS Layer ... dialogue, and repeating steps 10 and 11 above.