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Thot

How do Surveyors use Scaled Benchmarks?

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I've been benchmark hunting for a while now but there's something I've never understood. By far and away the majority of marks in my area are scaled. The coordinates/location given by NGS are normally not very close. The worst I've found was about 1500 feet off and 50-200 feet is common. So, they obviously can't be being used to survey property lines/boundaries. How are scaled benchmarks used by surveyors?

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You will find that almost every mark has either ADJUSTED horizontal coordinates or a ADJUSTED elevation, if not both. They are for different purposes, just as land (cadastral) monuments have their own purpose.

 

An elevation bench mark only needs horizontal coordinates good enough to find it. (Which is also good enough for the geoid orthometric corrections.) Its elevation needs to be very accurate in order to do flood certificates, drainage, highway slopes, etc.

 

The horizontal control marks (e.g. triangulation stations) are used as reference positions and their elevation may be unknown while still serving their purpose.

 

Neither kind is typically marking a land boundary in itself, although that may happen rarely, or boundaries may use them as offset reference marks.

Edited by Bill93

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Marks with just a scaled horizontal position are true "bench marks," that is a mark for which a very accurate height was determined. These marks are an essential part of our national infrastructure as they provide the foundation for all forms of topographic mappings, water management, flood insurance mapping, storm evacuation route planning - the list goes on and on. The height for these marks were determined by the process of geodetic leveling -

. This process does not have a horizontal component to it. Marks with accurate or "adjusted" heights were determined either by the surveying technique of triangulation, traverse or more recently GPS. Some marks have both adjusted vertical and horizontal values but they are a relatively small portion of the network. The scaled horizontal positions were as it states literally scaled from U.S. Geological Survey (USGS) 7.5 minute topographic maps. If the mark was shown right on the map it's generally possible to scale it's position to an accuracy of about 1-2 arc seconds (100-200 ft). Marks that were not shown on the maps required to cartographer to read the description and from that make a best guess estimate of the marks location - hence it is not uncommon to find their values in error by 200-600 ft and sometimes more.

 

It should be noted that with the exception of property boundaries defined by coastal water lines (e.g. Mean Low Water etc) geodetic control marks, both vertical and horizontal are only used in boundary surveys to provide and attribute for future use. With the exception of some of the state boundaries virtually no property in the U.S. is defined by coordinates. This may change in the future but it will require considerable changes in property law.

 

I should also note that GPS coordinates posted on this site for scaled bench marks have been harvested and updated nearly 70,000 marks in the NGS database which will contribute to improved recovery for others in the future.

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Thanks.

 

While hunting I see people with transits on tripods over marks with helpers with grade rods. I really don't know what they're doing but it's fairly common to either encounter them at the mark or see the evidence they've been there recently.

 

If the marks with adjusted horizontal coordinates aren't used to survey/locate property boundaries, what are they used for?

Edited by Thot

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I should also note that GPS coordinates posted on this site for scaled bench marks have been harvested and updated nearly 70,000 marks in the NGS database which will contribute to improved recovery for others in the future.

I submit recovery reports with coordinates and photos to the NGS using their DSWorld software.

Edited by Thot

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It's probably worth mentioning that land subsidence in our area makes vertical adjusted marks incorrect. I think before GPSs a few were used to measure the amount of subsidence by bringing measurements from 400-500 miles north. But, the height of vertical marks it pretty meaningless here.

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It's not that marks with good coordinates aren't used in boundary surveys, it's that they don't control the boundary that comes from the description of the parcel. The coordinates from either traverse or GPS surveys are most often used to as an attribute to the boundary corners as an aid for future recovery

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Thanks.

 

While hunting I see people with transits on tripods over marks with helpers with grade rods. I really don't know what they're doing but it's fairly common to either encounter them at the mark or see the evidence they've been there recently.

 

If the marks with adjusted horizontal coordinates aren't used to survey/locate property boundaries, what are they used for?

At the DOT we used to tie our road alignments into them. Part of US-2 in upper Michigan we ran traverses between triangulation stations (set new control) from; St Ignace to West of Escanaba, in the late 1970's and 1980's. All our plans were placed on the Michigan Coordinate system.

 

QL0603_MARKER: DH = HORIZONTAL CONTROL DISK

QL0603_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT

QL0603_SP_SET: TOP OF SQUARE CONCRETE MONUMENT

QL0603

QL0603 HISTORY - Date Condition Report By

QL0603 HISTORY - 1977 MONUMENTED MIDH

QL0603

QL0603 STATION DESCRIPTION

QL0603

QL0603'DESCRIBED BY MI DEPT OF HIGHWAYS 1977 (JPS)

QL0603'THE STATION IS ABOUT 0.62 MILES SOUTHWEST OF THE NEW COURTHOUSE IN

QL0603'MANISTIQUE, IN THE NORTHWEST QUARTER OF SECTION 13 T41N R16W.

QL0603'

QL0603'TO REACH THE STATION FROM THE NEW COURTHOUSE IN MANISTIQUE, GO NORTH

QL0603'ON MAPLE STREET (EX. US-2) 0.25 MILE, TURN LEFT ON ELK STREET AND GO

QL0603'WESTERLY ACROSS BRIDGE 0.7 MILE TO THE JUNCTION OF US HIGHWAY 2 AND

QL0603'STATE HIGHWAY 94. TURN LEFT ON US HIGHWAY 2 AND GO WEST 0.6 MILE TO

QL0603'THE JUNCTION OF US HIGHWAY 2 AND OLD STATE HIGHWAY 219

QL0603'(HARBORVIEW DRIVE). TURN LEFT ON OLD STATE HIGHWAY 219 AND GO

QL0603'SOUTHEASTERLY 0.3 MILE, TURN RIGHT AND GO 107 FEET SOUTHWEST TO THE

QL0603'STATION ON THE SIDE OF A SAND DUNE.

QL0603'

QL0603'STATION MARKS ARE MICHIGAN DEPARTMENT OF TRANSPORTATION SURVEYS

QL0603'ALUMINUM DISKS STAMPED---75001 1977---. THE SURFACE MARK IS SET IN A

QL0603'12 INCH CYLINDRICAL CONCRETE MONUMENT WHICH PROJECTS 2 INCHES ABOVE

QL0603'THE GROUND. IT IS 107 FEET WEST OF THE CENTER LINE OF A BLACKTOP ROAD

QL0603'(OLD M-219), 33.4 FEET NORTHEAST OF A WHITE PINE CLUSTER, 21.4 FEET

QL0603'SOUTHEAST OF A 6 INCH JACK PINE, 3 FEET SOUTHWEST OF A METAL WITNESS

QL0603'POST. THE UNDERGROUND MARK IS SET IN THE TOP OF AN IRREGULAR MASS OF

QL0603'CONCRETE 48 INCHES BELOW THE SURFACE MARK.

QL0603'

QL0603'REFERENCE MARK 1 IS A MICHIGAN DEPARTMENT OF TRANSPORTATION SURVEYS

QL0603'ALUMINUM DISK STAMPED---RM 1 1977. SET IN THE TOP OF A CYLINDRICAL

QL0603'CONCRETE MONUMENT THAT PROJECTS 1 INCH ABOVE THE GROUND SURFACE. IT

QL0603'IS 36.5 FEET SOUTHWEST OF A 6 INCH JACK PINE, 7.5 FEET EAST OF A WHITE

QL0603'PINE CLUMP, 3.5 FEET SOUTH OF A METAL WITNESS POST.

QL0603'

QL0603'NO AZIMUTH MARK WAS SET BECAUSE STATION MANISTIQUE AND MIDH 75002 ARE

QL0603'INTERVISABLE FROM THE GROUND.

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Most of them are National Geodetic Survey (NGS) points. They are either vertical or horizontal reference points or both. Before we had GPS the municipalities would set these markers for the local surveyors to use. We would use the datasheet to find the various markers. The bottom of the datasheets have the driving directions to get to the control points. The horizontal control points have latitude and longitude as well as State Plane coordinates. The benchmarks are control points that have established elevations. A lot of surveyors still use these control points on a daily basis.

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The datasheets that NGS provides have the coordinates on North American Datum 1983 (NAD83). Geocachers use World Geodetic System 1984 (WGS84). These are two totally unrelated datums and there is not a reliable conversion program. If you want to find benchmarks you will need to change the setting of your GPS to NAD83.

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The datasheets that NGS provides have the coordinates on North American Datum 1983 (NAD83). Geocachers use World Geodetic System 1984 (WGS84). These are two totally unrelated datums and there is not a reliable conversion program. If you want to find benchmarks you will need to change the setting of your GPS to NAD83.

 

If the mark is for elevation then the coordinates are scaled and the datum is of little importance. If the mark is a horizontal mark the the coordinates are very accurate and again the datum is of little importance. Almost all GPSr use WGS84 when you select NAD83 as the primary datum, so it makes no difference which you tell your GPSr to use.

 

Reading the description is more important than the datum you select.

 

John

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The coordinates are nearly always good enough to get you into the neighborhood, after which John is correct that the description is more important.

 

The difference between NAD83 and WGS84 is on the order of one meter (three feet) in most of the US. Since this is better than the reliable accuracy of a recreational handheld GPS, the difference isn't very important. NGS does have a program called HTDP that will give an estimate of the difference.

 

If you have a handheld that does display different coordinates for a recorded waypoint when you switch between these two datums, I'd like to hear about it since all the ones I've tried show no difference at all.

 

The difference between these two datums and NAD27 as used on the old topo maps is significant. It's around 50 feet in my area and larger in some places.

Edited by Bill93

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