Topic: Survey Question - Project to WGS 84

[Mark]

A single established benchmark and a bearing is this truly all that's required to convert between coordinates systems accurately?

Not trying to be picky on purpose but ... a benchmark is vertical control not horizontal. Control point is what you're looking for.

[mjfarrell]

and the more 'control' points one has that can be aligned with other KNOWN control points that are actually
correct in space (what ever the coordinate system) the more accurate any transformation will be

[nobody]

A single established benchmark and a bearing is this truly all that's required to convert between coordinates systems accurately?

Not trying to be picky on purpose but ... a benchmark is vertical control not horizontal. Control point is what you're looking for.

Thanks Mark...are they usually separate?

[cadtag]

dunno if it's standard practice, but usually the surveyors I get stuff from have twice as many control points (with0.00 elevation) as they do benchmarks.  EG if there are 4 points marked as BM, there are 8 marked con control

[mjfarrell]

A single established benchmark and a bearing is this truly all that's required to convert between coordinates systems accurately?

Not trying to be picky on purpose but ... a benchmark is vertical control not horizontal. Control point is what you're looking for.

Thanks Mark...are they usually separate?

Depends on the survey...

If one is placing control for aerial or lidar, most but not all control points are both horizontal and vertical control (reference) points.

If it is a site/topo survey there will typically be more control points, than bench marks.  Again typically the term 'bench mark' is for vertical datum, whereas
control points are there to adjust, and or calibrate to the individual site.

However sometimes these terms get tossed around as if they are interchangeable, yet they are distinct and separate from each other when used correctly. 

[Dinosaur]

I think this may be a difference in terminology rather than practice.
At least in the areas I have worked in where there may be a few hundred feet elevation difference throughout a project, the hierarchy of survey control is thus:
Control Points (CP) are points set by the surveyor usually within the bounds of the survey in a highly visible location that will remain undisturbed during construction and probably not usable to anyone other than the original surveyor as they are recorded ONLY in his notes.  They are set according to the project coordinate system and if there is an elevation attached it may be only very minimal accuracy and should probably be ignored.
A Temporary Benchmark (TBM) is the control point of the elevation world and are set again to the project coordinate system only with an elevation attached that is tied to probably some local vertical control that may or may not be tied to USGS.
A Registered Control Point is a permanent monument usually a Section or Quarter Corner that is tied into the appropriate State Plane Coordinate System complete with description of the monument certified and filed with proper authorities by the surveyor.  They usually have an elevation attached that is tied to the vertical control identified in the monument description.  Copies of all records of monuments used in a survey should be included when recording said survey.
At the top of this food chain is the USGS Tri Station and are recorded similar to the Registered Control Points.  They are permanent monuments set in prominent locations scattered throughout a local area and all Registered Control Points must be tied to at least two of them.  They are tied into the appropriate State Plane system with elevation set to at least 3 decimal points and may also have Latitude and Longitude in the document records.

[Dinosaur]

I left Benchmarks out of the above for a couple of reasons ... first because the dogs were having an urgent call of nature and second because the answer is not as permanently set as the monuments themselves ... which as it turns out may not be all that permanent itself.
Old school practice was to use about any "permanent" hard surface or structure in a highly visible area which was more than adequate accuracy for the times.  Eventually though, greater accuracy became desirable and then required.  About the same time the "permanent" was exposed as being not so much.  One of the favored structures for some reason at least in the Denver area for a (ahem) "BM" was the northeast bonnet nut of a fire hydrant.  Worked great until someone pointed out that those bonnet nuts might be removed and not replaced at precisely the same vertical location.  Of course this discussion was in response to the discovery of some elevation discrepancies and suddenly good enough was not any more.  Is no telling what may be found in any given area for a real Benchmark and they may not even tie into USGS elevations depending on local regulations.  Here in Kansas City, certain areas must be tied into datum derived from some theoretical point in the Missouri River channel with conversions to and from USGS datum.

[nobody]

I left Benchmarks out of the above for a couple of reasons ... first because the dogs were having an urgent call of nature and second because the answer is not as permanently set as the monuments themselves ... which as it turns out may not be all that permanent itself.
Old school practice was to use about any "permanent" hard surface or structure in a highly visible area which was more than adequate accuracy for the times.  Eventually though, greater accuracy became desirable and then required.  About the same time the "permanent" was exposed as being not so much.  One of the favored structures for some reason at least in the Denver area for a (ahem) "BM" was the northeast bonnet nut of a fire hydrant.  Worked great until someone pointed out that those bonnet nuts might be removed and not replaced at precisely the same vertical location.  Of course this discussion was in response to the discovery of some elevation discrepancies and suddenly good enough was not any more.  Is no telling what may be found in any given area for a real Benchmark and they may not even tie into USGS elevations depending on local regulations.  Here in Kansas City, certain areas must be tied into datum derived from some theoretical point in the Missouri River channel with conversions to and from USGS datum.

Thanks Dinosaur