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With at least some USGS quad maps, maps that overlap into adjacent UTM zones continue to use the same false easting (rather than start at zero in the middle of the map).

Is this a required standard convention in the UTM specification or merely done for convenience?

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    I don't know if the USGS Quad maps have a specification that discusses this, I haven't had much to do with them. I'd say they do it for convenience. I used to make 250k maps, with a zone change on the ephemera it was a right pain to add a new grid/graticule and project the data. Nowdays I would use the zone for the majority of the data and continue the same zone over an edge - so long as you don't go too far the distortion isn't too bad. If you're going well into an adjacent zone it's time to think about non-UTM projections like Albers Equal Area or Lamberts... – Michael Stimson Sep 3 '15 at 23:24
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    I was told many years ago, in South Australia where Port Pirie straddles a zone boundary, that as long as the data did not go further than 30 minutes into an adjacent zone then it was perfectly acceptable to treat that adjacent data as if it was in the main zone and project it into that false zone. – PolyGeo Sep 4 '15 at 1:44
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I worked on updating some USGS quads, back in the 90s. It seemed like most of the style guidelines were published internally, long before the Internet, and never made it online.

It's fairly common to show two sets of State Plane Coordinate System grid tics on quadrangles that are near a boundary between two zones. With UTM zones, it isn't such a big issue. The amount of scale distortion within a UTM zone was designed to be less than 1 in 1000. At the edges of the zones, it's 1 in 1000 (at the equator). The distortion increases beyond the boundaries of the zone, but you can usually go about 40 km into an adjacent UTM zone before the distortion becomes a concern. (I do have documentation on that somewhere).

I would imagine, based on the State Plane Coordinate System example, that quadrangles on a UTM zone boundary would have grid tics for both zones. I would be surprised if that continued more than 1 or 2 quadrangles east or west of the zone boundaries.

I'm not entirely sure what you mean by "start at zero in the middle of the map". Each zone would use the easting value measured from its own central meridian, which is set to 500,000 meters. At the zone boundaries, you'd have higher eastings measured from the zone to the west, and lower easting values measured from the zone to the east, but you wouldn't go far enough to get an easting of zero.

  • Your last paragraph gave me a slap-in-the-face of clarity. I always thought of UTM zones as being measured off an imaginary baseline 500,000m west of the central meridian and forming the western boundary of the zone (which in hindsight would never work, UTM zones have curved boundaries and are always narrower than 1,000,000m). I need to think of them like pH values where 7 is the center. – saltface Sep 4 '15 at 22:06
  • FYI, UTM scale distortion is designed to be 1 in 2500 (0.9996 on the central meridian). It's actually a little worse than that at higher latitudes. – mkennedy Sep 8 '15 at 23:22
  • Interesting question and answer. How is this handled in a GPS unit when set to UTM? – Jakub Sisak GeoGraphics Sep 8 '15 at 23:55

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