why are benchmark coordinates listed as not being very accurate

[georapper]

i thought with all that expensive survey equipment, 10's of thousands of dollars worth for survey grade equipment, why do all the benchmark coordinates list a disclaimer that they are not accurate? you would think that survey grade equipment could be more accurate, even with NAD83 datum.

 

anyone know why?

 

Creativity Within The Bounds Of Conformity

[survey tech]

Standards of accuracy are constantly changing due to technological advancement. What is considered accurate today may be considered sloppy in the future, just as the most accurate work of the past now looks less impressive by current standards. All NGS work is state-of-art at the time its done, but is subject to ongoing adjustments. The North American triangulation network is still considered the finest such network of its size ever created.

[rusty0101]

Another thing to take into consideration is that it takes a significant amount of time to make sub-centimeter measurements.

 

On top of that there is a bit of randomness between different pieces of equipment. One of the sigs you will read if you go through the forum points out that someone with a single GPS is never lost, someone with two GPSrs never knows where he is.

 

As a last point of note, any GPSr that I may happen to use, has nothing like the accuracy of a surveyors GPSr. If I take a data sheet and expect my GPSr to take me to that mark, I will be sorely disapointed. My display may report that I am within 12 feet of the mark, but that last 12 feet means a circle of diameter 24 feet, or about 80 square feet of area. (if I happen to be lucky)

 

-Rusty

[Z15]

Well lets see if I can make it simple but it may not seem simple to those that are unfamilar with surveying operations.

 

On this site and in this forum the term Bench Mark is used as a generic term for all control but the more precise meaning is; A bench mark is a point for which an elevation has been determined.

 

When a Bench Mark is established usualy there is no precise determination of position, only a hand plotted position of a field map is done by the recon team that recovers the mark or establishs it and only if its not already a horizontal control mark. Reason: They are only determining elevation and position is not necessary. The level party does not make position determinations, that requires more precise and time consuming activites.

 

No with that said, you will find marks that were established as Bench Marks that later were also used for horizontal control determinations and vice-versa. Some horizontal were used by the level party.

 

It all boils down to what the scope of the survey was to start with. With modern day GPS things are starting to change rapidly. They are working on perfecting the use of GPS for determining elevlations with relative accuracy.

 

This is a very complex topic (GPS elevations) and many papers have been written on it and available on the NGS website.

[+rogbarn]

It is my understanding that some points are elevation controls only and precise latitude/longitude coordinates were never established for them. In other words, what elcamino said. For those points that are horizontal controls, even old ones, the coordinates should be so close that our consumer grade GPSr can't tell any difference. This is why looking at the original datasheet to determine the coordinate precision is so important. Unless you are like me and go benchmark hunting using only the directions in the history and no GPSr.