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trmcconn

Marks in polar regions

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Have surveys been done near the earth's poles, or to ask it another way, what is the distance to a pole from the nearest survey mark?  I have a very indirect reason for asking. AZ marks are usually listed on datasheets with an exact azimuth and approximate distance. My understanding is that azimuth is measured from the surveyor's meridian to a curve on the surface of the earth connecting the surveyor and the AZ mark. Which curve, exactly? Is it a great circle, or a rhumb line (line of constant heading)? In temperate latitudes it makes essentially no difference at the typical distances of AZ marks, and probably surveyors in these regions just measure azimuth from meridian to a line of sight to the AZ mark. But near the poles it could make a difference, so if surveys have been done in polar regions the profession may have some standards that address the issue (I'm not a surveyor.) The deeper reason for my question is that I'm working on a program to generate polygonal gpx tracks connecting given points. The sides of the polygon can be either great circles or rhumbs. The program has an option to list vertices or heading/dist from a given point and I'd like to get things "right". 

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Azimuth marks were sighted from triangulation stations. Line of sight would use the great circle. Fortunately, that is going to be easier to compute than rhumb.

There probably are few if any such marks far enough north that it will make a serious difference, especially over the distance of an az mark.

For moderate distances spherical calculations will be close enough. For miles to hundreds of miles, depending on desired accuracy, you need to work on the ellipsoid model of the earth. For precise calculations you need to consider height above the ellipsoid, changing your local earth radius and thus the length of a degree.

NGS has a toolkit on their site that helps with calculations. You should look at FORWARD and INVERSE.

 

Long boundary lines run on the ground (e.g., public land surveys dividing up a state into mile-square sections) were run as a series of sights of whatever length worked, up to a mile, nearly always N, E ,S, or W.  Over a longer distance they were thus close to thumb lines.

Some state boundaries were specified to be a parallel of latitude, and thus a thumb line.

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There are no systematic surveys performed near the polar regions.  The U.S. Geological Survey (USGS) has set a survey marker at the South Pole ever year for several decades for the purpose of estimating tectonic motion.  Since the polar regions do not belong to any one nation their are no national surveying requirements.  Because of the dynamics of ice motions and surface marks would be disturbed/suspect in fairly short order.  The use of azimuth marks was started by the U.S. Coast & Geodetic Survey (USC&GS) in the early 1930s (32-33) as a way for local land surveyors who lacked sophisticated geodetic instruments to be able to connect their local surveys to the national coordinate framework.  The azimuth from the triangulation station to the azimuth mark (typically about .25-.5 mile away) are always computed with respect the reference ellipsoid used for the coordinate system - either Clarke 1866 for the North American Datum of 1927 (NAD 27) or Geodetic Reference System 1980 (GRS 80) for the North American Datum of 1983.  If you are using satellite positioning systems and want to "get things right" then all positions and directions should be computed with respect to a reference ellipsoid -- GRS 80 is the current international standard and likely to be so for many years to come.

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On 4/15/2020 at 9:39 AM, DaveD said:

The azimuth from the triangulation station to the azimuth mark (typically about .25-.5 mile away)

 

I don't know wy, but I find this hilarious:

RM1 is 13.5m north of station.

RM2 is 11.5m north of station.

AZ is 101.3m north of station.

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