Orienteering USA is now in a partnership with a major provider of aerial images across the United States, Pictometry Online (POL). Through this arrangement, mappers working on behalf of Orienteering USA clubs may download high-resolution, georeferenced aerial images for use in creating new maps as well as for updating existing maps. This resource is made available through a cost-sharing arrangement between Orienteering USA and each club; one hour of online access is charged to each club at the nominal rate of $18/hour, regardless of how much imagery is downloaded. Club officials should email Orienteering USA Mapping Coordinator Greg Lennon [greg underscore lennon at orienteeringusa dot org] to obtain accounts for their users.
Approximately 85% of the places where people live in the US are covered. Generally, you'll find better coverage of areas nearer towns and cities, and poorer coverage in unpopulated areas. The coverage continues to grow since more images are collected every year. An example of the coverage over the East Coast from Washington DC north is shown below; the dark green areas indicate where coverage is available.
There is no charge to scan the areas covered, so you can check that out before spending a cent of your club's money. You need to establish an account in order to get the log-in information, though, in order to get access to the online system.
Many areas have 2 or 3 "cycles" worth of photos; a cycle is every 2 or 3 years depending on the area, so the most recent photo is currently likely to be from either 2009,. 2010, or 2011. In other words, new photos are taken of almost all areas every 2-3 years. Most photos are taken during leaf-off season(s), and when the ground is not obscured by snow. Dates of each photo are available.
For most areas, orthophotos (i.e. looking 'straight down') are available in addition to oblique photos (bird's eye) taken from the 4 principal compass directions (N, S, E and W). While orthophotos are obviously of primary use when making basemaps or updating existing maps, the oblique photos can add information, especially in mixed use areas and towards map edges. In terms of resolution, POL uses the terms "neighborhood" and "community" for their images; when exporting, in other words, at 100% scale, neighborhood images are 4 to 6” per pixel and community images are 12” per pixel. Community imagery is usually available for all of a county if any images are available (for that county), while neighborhood images are mainly where the population centers are.
Google images are great and by all means use them whenever possible. However, they are often taken during leaf-on seasons, and they require additional work to georeference. NAIP 1m images are almost always leaf-on images, and although they are georeferenced, their resolution is 9 to 36 times lower than the resolution of POL community or neighborhood images, respectively. One of the best sources of high resolution public domain orthoimagery is actually the USGS Seamless Data Warehouse; head to the HRO section. So if that's enough, great. Nonetheless, if POL images are available for an area you are mapping, and especially if your maps are georeferenced (which we strongly encourage), the few dollars your club pays for a set of high quality images is likely to save substantial fieldchecking time and effort.
Lat/Long coordinates are (only) represented in decimal degrees (DDD.ddddd; example: 38.89765, -77.036506); orthoimages are oriented to true north. Orthoimages are exported as JPEG files and you must download an (ESRI) World file to accompany each JPEG. You will probably need to convert the images to other units and/or formats after you export them from POL as needed. While there are several programs that can be used for such conversions, Global Mapper is one that is often used (such as to convert DDD.ddddd JPEGs to UTM GeoTiffs, which most mappers find easier to import and use as background images in OCAD), and it's available at a discount to nonprofits. [Contact Global Mapper directly for current nonprofit discount.]
Practicing in the 'Training Area' and checking image Coverage are free. Looking at "real imagery", whether or not you export it, is charged in increments of 10 minutes, at $3 per 10 minute increment ($18/hour). Each increment is rounded up to the nearest 10 minute increment, so you will be charged $3 whether you are on for 1 minute or 9 minutes. Downloads are fairly quick assuming you have a high speed connection, so almost all time is spent deciding which image of the ones available is the one you really want. By the way, there are also free training videos, classes, powerpoint presentations, and online help material available to registered users.
Just contact Orienteering USA Mapping Coordinator Greg Lennon, preferably by email [greg _ lennon at orienteeringusa dot org].
Right here (below, in order). As mentioned in O/NA, these three represent the same (small) section of a national park being mapped. The first two images are photos taken in summer, freely available from the US Dept. of Agriculture; the first is from the National Agricultural Imagery Program (NAIP), at a per-pixel resolution of 1.0m, and the second is from the Natural Resources Conservation Service (NRCS), at 0.5m resolution.
The third image is the Pictometry image taken in early Spring and at a per-pixel resolution of less than 0.2m.
Coverage Map mentioned in POL FAQ #1: