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Strange crosses in USGS Digital Ortho

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  • Kaluva
    started a topic Strange crosses in USGS Digital Ortho

    Strange crosses in USGS Digital Ortho

    39.49989843015873
    -93.00010636436416

    What are those crosses? Are they actually there, or are they just artifacts from image processing?

  • withak
    replied
    The times I have seen them on the ground they have been in the center of intersections or on top of some utility so they can use the location from a survey that already exists. Even if they aren't in an urban area it would make sense to at least put them in a road so you don't have to get permission from the landowner to survey them.

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  • bobg
    replied
    crosses in the middle of fields.

    Hey Woodse, you were'nt tromping around in some farmer's hayfield were you?

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  • withak
    replied
    There is an alignment error in the USGS topo layer at that exact same spot too. Maybe it's a black hole.

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  • withak
    replied
    However they got in the original images, the ghosted crosses are probably an artifact of the orthorectification process.

    They look more like something added to the image afterward to me. Crosses actually put on the ground for reference would have to get surveyed at some point, so they wouldn't be likely to appear out in the middle of a field like that.

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  • bobg
    replied
    re: cross marks

    The crosses are a part of the orthophoto. But I'm not familiar with aerial photo techniques, so I'm really not sure if they were put there as they processed the film in the labs or as someone pointed out earlier, someone on the ground actually put white tape down at regular grid intervals for a point of reference. I can imagine in the days before GPS technology that tape on the ground was the only reliable evidence of grid positioning. All I know for sure is I've seen these alignment crosses in DOQ's I was viewing in another GIS program. I'm curious as to why a second un-aligned cross sometimes appears "ghosted"?

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  • withak
    replied
    Processing into WW tiles doesn't create marks like that.

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  • bobg
    replied
    White crosses

    I've seen these crosses in many U.S. Digitial Ortho Quads. I'm assuming its a tile alignment mark exposed onto aerial photographs during film processing of the tile strips, much like alignment marks on the camera-ready copy in the publishing industry.

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  • woodse
    replied
    When I volenterred for the Park service we would sometimes put down large white crossses for arial surveys. It could be that our It could be an image artifact.

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  • Guest_Jim_*
    replied
    I think I found what Kaluva is talking about.

    worldwind://goto/world=Earth&lat=39.50002&lon=-93.00004&alt=256
    Attached Files

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  • withak
    replied
    Can you post a screenshot? I'm not getting anything from the server to look at.

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