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t8r

7 1/2 minute map corners

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On terraserver topos and photos there are these little crosses that correspond to the corners or 7 1/2 minute series paper topo maps. I just noticed there are 2 of them in a photo of the area near me. A closer examination of the topo revealed what appears to be the remains of one that may have been erased. the erased one corresponds to the fainter one in the aerial photo. Is this possibly a nad27/nad83 thing? any ideas?

http://www.terraserver.microsoft.com/image...Y=23742&W=3

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While the marks may have been placed to indicate the location of the quad sheet corner (and its corresponding Lat/Long) I would doubt that the marks are to show two different datums. I could not find any other 'double marks' in the little bit of panning I did on Terraserver. If the two marks were indicative of two different datums, there would be multiple pairs of marks as a datum shift is fairly consistent within any given area. They may be just working marks, left-over from the photogrammetrist who did the photoprocessing.

 

- Kewaneh

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I have paper 7.5 minute maps (unfortunately none later than 1982) that have dashed corner marks slightly different from the solid line corners, and say things like:

 

"1927 North American Datum. To place on the predicted North American Datum 1983 move the projection lines 4 meters north and 14 meters east as show by the dashed corner ticks"

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Many USGS quads were produced with a second set of marks at the estimated NAD83 positions for the corners. Some of these were computed based on estimated shifts before NAD83 was actually finished and may not be exactly correct but give a good visualization of the differences. In DRG images made from the paper quads these are often hard to clearly distinguish. I have even seen a DRG which was produced on NAD83 and the offset corner marks were NAD27, so never assume.

 

Terraserver imagery derives from the National High Altitude Photography program which generates the USGS DOQQ's.

 

DOQQ's have two sets of tic marks. One which is usually a solid cross is the NAD83 value, and the other which is dashed is NAD27. Because QQ means quarter quad these can often be found at even values of 3 minutes and 45 seconds in the Terraserver data. There is actually meta data that was distributed with the images originally which describes what the marks are.

 

- j wahl

 

I have paper 7.5 minute maps (unfortunately none later than 1982) that have dashed corner marks slightly different from the solid line corners, and say things like:

 

"1927 North American Datum. To place on the predicted North American Datum 1983 move the projection lines 4 meters north and 14 meters east as show by the dashed corner ticks"

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I knew someone would have the complete invo.

The one cross appears to be dashed. I thought possibly faded

Thanks jwahl.

:)

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