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Papa-Bear-NYC

Scaled coordinates or elevations: when?

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We are familiar with the concept of "scaled" coordinates and/or elevation values, and with the innacuracy of same. Many newcomers to the field are surprised when they learn that some of the data published in the datasheets is approximate and not based on direct measurements.

 

A series of related questions:

 

1) Why does the NGS bother publishing these data? Perhaps to help surveyors find the marks? This does not explain scaled elevations on triangulation stations.

 

2) When did the NGS start doing this? Did they do it when they were a paper organizatin, i.e. before the datasheets were generated electronically? When was that - 1950s?

 

3) Is there any methodology defined for how it is done?

 

4) When new maps are published, or when the datums were changed in the late 1980s, were the scaled coordinates and elevations "adjusted"? (An amusing thought). So if a tri-station was assigned a scaled elevation from a spot elevation on an old USGS map, was that updated when newer maps came out?

 

I would love to see any official statement on this topic, or any information from old-timers (or new-timers) at the NGS.

Edited by Papa-Bear-NYC

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Curious. I was just looking at the pages for one of the hundred or so stations set in concrete in clay tile pipes on the banks of the Passaic and Hackensack Rivers in 1913, and never seen again. In this case KV4159. Horizontal coordinates were adjusted by NGS in 1999. Orthometric height is scaled from a topographic map. In this case on the filled in dock of the Naim Linoleum Company. I have not located any history of the Naim Linoleum Company, but the disk was reported lost by Coast and Geodetic Survey in 1933. Google Earth does not show a dock at this location, which is now part of River Bank Park. But we now have accuracy to five decimal points on the seconds, for a disk that has probably been missing for ninety three years. I'm not sure that I understand the reason for doing this either.

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Its hard to understand if you are not in that profession. A survey mark is always a survey mark. Just because its one persons opinion it lost is inconsequential. They never will throw out the data and thus it will likely always be part of some adjustment. There don't have time or $$ to determine if every point if the DB still exists, they treat it like it does. Thats my interpertation, when we could not come up with an answer, we would just say; "It does not need to make sense, its government work!"

 

We had othersayings to:

 

Why do it right when we can do it wrong.

 

You are educated beyond your intelligence.

 

An engineer is someone who, everyday learns more and more about less and less until he knows everything about nothing.

Edited by Z15

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Please bring this thread back to my original questions. It's called staying on topic. The new discussion may be interesting, but I would really like to know the answers to my original questions.

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NGS began constructing the foundation of the current database in the mid-1970's. At that time the data for stations was published as separate horizontal and vertical control documents. As part of the data frame it was decided that all monumented control points would have a position and an elevation, even if one of those values needed to be scaled from a map. At that time NGS had a small cartographic section that was tasked with plotting every station in the network (more than 900,000) on USGS 7.5 minute, 1:24,000 topographic sheets when necessary to get the elevation or position. In some cases this was easy because USGS already showed the mark on the topo map. In many others, the cartographer was required to read the description and follow the directions along the map to plot it as accurately as possible. Because the subjectivity of this process NGS decided to label the quality of scaled horizontal values to +/- 6 arc seconds. Since points with a known latitude and longitude were easier to plot we were generally confident to be able to scale the elevation for points that required that component to within 1/2 a contour interval. Having at least a scaled value for bench marks provides spatial information for computer mapping software that allows us to accurately assess the density of control stations in a given area and provides approximate information for surveyors to locate these marks. Scaled elevations on horizontal stations provides a check on the reduction of certain types of survey measurements that are fundamental to land surveying operations.

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Thanks Dave. As always you are the "answer man". The one point missing was the question of "adjusting" scaled values when the datum changes. But I guess with an accuracy of +/- 6 seconds and of 1/2 a contour, that issue is largely irrelavent.

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EXCELLENT description. This was something we had discussed a long time ago, but Dave put it in historic context and described the actual process wonderfully. Although I knew the scaled stations were taken from topo maps before, now I know why and exactly how!

 

Thanks Dave.

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... and most importantly WHEN. I have seen several cases where the road with the number given in the to-reach had been moved miles away from the original location, and the scaled coordinates were on the new road instead of the old one. Without having detailed historical maps to help, it would be impossible to get all of those right from the more recent topo.

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