Back in 2006, several state, federal, and private groups formed the Statewide Ortho Imagery Partnership, which funded the acquisition of high-resolution (6 inches per pixel) orthoimagery. Partly because the US Geological Survey was one of the funding partners, the imagery was put into the public domain. The images were recorded between fall 2007 and spring 2008, so most of it is leaf-off. On top of that, the georectification seems excellent. In every place where I have multiple GPS traces to consult, the imagery matches up extremely well with the traces, and there seems to be very little distortion in the images. I haven't found any documentation of error margins for the imagery, but I'd be surprised if the error were greater than a meter.

My mapping has benefited enormously from the availability of this imagery. The rectification means that I can largely rely on the images for precise feature alignment and focus my surveying on things not visible in the imagery like addresses and names of things. The completeness of coverage has allowed me to map things that are either difficult to survey in person (like streams that run through remote areas or private property) or impossible (like much of the high voltage power line network in the state).

As time passes, however, the imagery becomes outdated. I've added stuff to OSM that's only in NAIP imagery (which is lower-resolution and not at well rectified) or that I've had to align solely from GPS traces or correlation to older features. I know there are power lines that have been added since the imagery was flown (I've sporadically watched a extension to the substation near Reisterstown as it's been constructed). That's why I was happy to have found a press release from Axis Geospatial, the company that provided the 2007/2008 imagery, saying that they reimaged the Eastern Shore in 2010 and will be redoing the rest of the state in 2011. Based on what I can tell about the availability schedule from the last dataset, it will probably be 2012 before all the updated data is available in some form, and it may take longer to get it onto the USGS's WMS, which is what I use for my mapping. Still, I'm quite excited about the prospect of updated data.