> Seriously though (& not arguing :-)), "a semi-trailer truck poking out of a > roof. Pavement staining and tire marks from fork lifts" wouldn't really be > enough to say definitely whether you're looking at a roof over an open area > or an enclosed building, would it? That truck could be poked out of the > doors of an enclosed loading dock, & the forklift could be doing the same?
Usually, no single thing will lead to a conclusion. The identification 'keys' basically indicate the other things depending on the context one should look for. Hey, it could be a short truck, too :-) Preponderance of evidence. Which is why it's useful to use multiple imagery sources. Bing and Google Maps are aggregators of imagery - they license it from other companies, at some price, at various resolutions, and what shows can change over time - urban areas get updated very frequently, not so much when you get out into the sticks. Even this isn't an absolute, by some freak of availability we observed a bear on the road of my brother's property on the Front Range in Montana. Also, the orthorectification can sometimes be crap ( adjusting for terrain, etc. ) > Yes, if you've got slanting or night time imagery that may help, but I've > never seen it in the areas I map in :-(> You can add your own sources to JOSM, the presets are the more or less globally useful services. Sometimes, you don't need to actually add it to JOSM to assist, like the building overhang issue. Our county has phenomenal lidar point data available ( 6"), I can make out the location of the picket fence on my front lawn. Our state flies regular oblique photography to monitor coastal conditions, also. There is an upfront effort of finding what's available, but once you have the endpoints and sources for your area of interest, it can make things easier. For instance: JOSM displaying the USGS 3DEP ( derived from Lidar surveys ) http://bit.ly/2Rjb8LO - in some areas it has 1 meter resolution along with slope aspect, etc. https://www.usgs.gov/news/new-elevation-map-service-available-usgs-3d-elevation-program and for thermal ASTER ( Advanced Spaceborne Thermal Emission and Reflection Radiometer) https://bit.ly/2FoslNE - not great, it's a fairly old platform and varies between 15 to 90 square meters per pixel, but I had it handy. > On the subject of clarity of images. Was mapping the other day (using iD), > marking buildings in an industrial area. As I said, the photo's weren't the > clearest, but I was also peering through the purple haze of the mapped > area=industrial, which certainly doesn't help matters either :-( Yeah, we have 308 cloudy days a year. Which underscores the value of having sources that have multiple times available. Leaf on and leaf off is also useful. If Bing's blender picked a day when an inversion layer was occurring, it's nice to pick another day. Night imagery is rare. The gold standard for this is SAR ( synthetic aperture radar ) https://earth.esa.int/web/eoportal/satellite-missions/t/tsx-ng but it isn't really mainstream yet, may never be for the general public for obvious military reasons. Many countries are launching these platforms, though, so it may be like the 'fuzz' on GPS accuracy, once everybody has it, they'll open it up. Depending on your level of commitment, it's useful to have some minimal skill with QGIS, just for these discernment purposes. Then you can do things like pansharpening ( https://www.geoimage.com.au/images/services/pan_sharp_bne_qb.jpg ), contrast adjustment, etc. very simply just by altering the layer transparency - tasks that aren't possible in JOSM. Michael Patrick Data Ferret
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