USGS High Resolution Orthoimagery
The United States Geological Survey (USGS) High Resolution Orthoimagery (HRO) is a set of high resolution aerial photos that are much better quality than Yahoo's imagery. The photography is mostly but not entirely in the public domain; see below for how to ensure that you only use public domain data. Finally, the images are available via WMS, so they can be easily incorporated into JOSM, and with somewhat more difficulty into Potlatch and Merkaartor.
As of February 2012, imagery is now available as a new map service at:
- ArcGIS Services Directory - info
- wms - service
The data is generally available on a county-by-county basis. You can check the license for a county's imagery at the USGS's Orthoimagery List. If it's not listed as public domain, it can't be used.
Alternately, the WMS for each state includes footprint layers as well as image layers for each county. If the footprint is colored blue, the data is public domain, while the footprints for restricted data are colored yellow. (The same color coding applies to the displays for the "State & County Ortho Index" and "State & County View Only Ortho Index" on the National Map Viewer.) The orthoimagery list above is probably the best resource, though.
Tiled vs Seamless
USGS provides imagery via two different systems, the Tiled Data Distribution System (TDDS) and the Seamless Data Distribution System (SDDS). In general the best imagery available is provided through the Seamless service, and older imagery is archived in the Tiled service. However, this is not always the case (such as in Northern Virginia). The High Resolution Orthoimagery List linked to above shows which system a given imagery set is available on, either Tiled or Seamless. If the imagery is on the Seamless system, then generally you can access the imagery via the WMS services as described below. If it is in the Tiled system, the imagery is more difficult to obtain.
Tiled - GeoTIFF
Hopefully this section will be expanded in the future to assist others in obtaining this imagery, however for now here is some relevant information:
- USGS Application Services - can be used to programmatically obtain download URLs for tile packages
- USGS EarthExplorer - can be used to download individual tiles; pick a point and then choose Aerial Photography->High Resolution Orthoimagery
- The downloaded file is a ZIP file containing a GeoTIFF and associated metadata
- The GeoTIFF can be split into TMS-style tiles using GDAL2Tiles, and then used in JOSM or Potlatch
Seamless - WMS
The WMS capability URLs are available from the USGS's Seamless server's WMS Access List. Each state has its own WMS. The photos are generally separated by county, though not all counties are present for all states, and some counties are further split into multiple layers. The layer names also indicate the year in which the photos were taken. A given WMS might have multiple layers covering the same region where the layers differ in the age or resolution of the photographs. There are also footprint layers, which merely indicate the extents of the corresponding county's photography (plus they're color coded as to the layer license; see above), and there are occasional metadata layers. If in doubt about which layers to use, it's probably easiest to load the capabilities URL into QGIS (or other GIS software) and see what's in each layer.
Layers later in the list take priority over layers earlier in the list, so if you need to combine layers with different qualities, the better layers should be listed last so their data is visible where they overlap with lower quality layers.
The following is an example URL for the imagery for Baltimore and Baltimore County in Maryland. There's one layer for Baltimore City, and two layers for Baltimore County. All three are public domain, so we can use them. The resulting URL would be as follows, but without the line breaks inserted for readability:
http://imsortho.cr.usgs.gov:80/wmsconnector/com.esri.wms.Esrimap/ USGS_EDC_Ortho_Maryland?version=1.1.1&request=GetMap& format=image/jpeg&layers=BaltimoreMD_0.5ft_Color_Mar_2007, MD_BaltimoreCounty_0.5ft_Color_Mar_2007_01, MD_BaltimoreCounty_0.5ft_Color_Mar_2007_02&
JOSM supports transparent image layers, which can allow you to have multiple WMS URLs loaded, only displaying the lower ones when the upper ones don't have any data. To use that, replace the format=image/jpeg in the WMS URL with format=image/png&transparent=true.
One potential use for this is to have a URL for footprint layers on top of a URL for photos, to show you when you've moved into a region with other photography available. (The advantage of using the footprints over the actual other photography is that requests for photos outside the WMS layer's bounding box result in error tiles, while requests for footpints in the same place result merely in completely transparent tiles.)
Using imagery in JOSM
- wms link (as of February 2012)
Previous imagery location
Some imagery is still available in the old USGS wms services, though some has gone missing or is degraded quality.
Now click Get Layers, and choose the layers you wish to use, then click OK and the layers will no be available in the Imagery menu.
If you use these photos for your mapping, tag them source=USGS Ortho.