Suitable for GPS

[Z15]

For those that are sending reports to NGS for inclusion into the data base.

 

They want to know if a site is suitable for GPS or not. Just because a site is open to the sky, it may not be suitable for GPS. You have to take into consideration of the location. For instance a disk set into the aboutment of a bridge may be clear to the sky but it may be next to impossible to set a tripod on because of local obstructions.

 

When you encounter these points that are not easy to setup on, it should be noted in the description.

 

We searched the database for GPS usable marks and when we went to one we found out we could not use it withour special equipment. We had to get a bipod (did not have one with that team), 2-3 5 gallon pails of sand and sand bags to steady the GPS receiver from the strong winds coming off Lake Michigan. Also had to use 2 Theodilites (Survey Instrument) setup up at 90 deg to each other to monitor the plump of the bypod for the 5.5 observation time required by NGS. Took 2 men instead of normal one person.

quote:

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QK0547

QK0547 STATION RECOVERY (2001)

QK0547

QK0547'RECOVERY NOTE BY MICHIGAN DEPARTMENT OF TRANSPORTATION 2001 (GEK)

QK0547'STATION IS LOCATED ON THE PROPERTY OF MICHIGAN LIMESTONE OPERATIONS,

QK0547'INC., PERSONAL PROTECTIVE EQUIPMENT, AND ENTRY PERMIT REQUIRED (NOT

QK0547'SUPPLIED BY MLO, INC.). MARK IS DIFFICULT TO OCCUPY W/O SPECIAL

QK0547'EQUIPMENT, DUE TO A 3.5 FT HIGH WELDED STEEL RAILING 0.5 FT N OF THE

QK0547'STA., AND IS ON THE EDGE OF A BREAKWALL DROPPING OFF TO THE WATER.

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Another example

quote:

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RL0728

RL0728 STATION RECOVERY (1994)

RL0728

RL0728'RECOVERY NOTE BY NATIONAL GEODETIC SURVEY 1994 (CFS)

RL0728'NOTE--NOT SUITABLE FOR TRIPOD SETUP. BRIDGE TO BE REMOVED IN 5 YEARS.

RL0728'STATION IS LOCATED IN QUAD 460891, IN ONTONAGON, ALONG STATE HIGHWAY

RL0728'64, ON THE RIGHT-OF-WAY, IN THE TOP OF THE NORTH END OF THE SOUTHWEST

RL0728'WINGWALL OF THE HIGHWAY BRIDGE OVER THE ONTONAGON RIVER, AND IN THE

RL0728'SOUTHEAST 1/4 OF SECTION 25, T 52 N, R 40 W, ONTONAGON TOWNSHIP.

RL0728'OWNERSHIP--STATE DEPARTMENT OF TRANSPORTATION. TO REACH FROM THE

RL0728'JUNCTION OF STATE HIGHWAYS 64 AND 45 ON THE WEST SIDE OF ONTONAGON, GO

RL0728'SOUTH ON HIGHWAY 64 FOR 0.2 KM (0.10 MI) TO THE FAR END OF THE HIGHWAY

RL0728'BRIDGE AND THE STATION ON THE RIGHT. STATION IS 11.8 M (38.7 FT) EAST

RL0728'OF THE NORTHEAST CORNER OF A CHAIN LINK FENCE, 7.0 M (23.0 FT)

RL0728'SOUTHWEST OF CENTERLINE OF HIGHWAY 64, 5.1 M (16.7 FT) NORTH OF THE

RL0728'SOUTH END OF WINGWALL OF BRIDGE, AND 0.5 M (1.6 FT) BELOW LEVEL OF THE

RL0728'HIGHWAY.

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[This message was edited by elcamino on April 13, 2003 at 12:14 PM.]

[rusty0101]

Well, I'll bite, but only because I feel properly baited.

 

I will agree that a bridge abutment does seem unlikely to be a good point to be considered suitable for GPS readings. At the same time I have my doubts about it being a good spot for a "light" or other most other survey equipment.

 

My prime candidate is 2781AH with pid PP1048. I suspect that the railing to the immediate south of this mark would make it difficult to set a tripod to position directly over this mark. (the clipboard in the last picture is approx 9.5 inches wide, the water bottle another 4 inches perhaps, then the railing. I personally would not consider this a good candidate for a GPS reading for other reasons, such as the huricain fencing to the immediate north (this is the end of a foot bridge over I-94) and the high rise appartment complex across the street to the south. Both provide obstructions that reach above 15 degrees above the horizon.

 

Oh, if you haven't brought along a sextant, or even a protractor, string and nut to make a improvised sextant, you almost always have a field expedient way to measure 15 degrees above the horizon. This is not exact, by any means, but is often handy when you don't trust your watch, etc. If you hold your arm out, and bend your wrist so your palm is towards you, and the bottom of your hand/little finger is on the horizon, the top of your palm is approximately 15 degrees off the horizon. This makes a makeshift clock in that 15 degrees is 1/12 the the arc across the sky, meaning that if the sun is above your hand when placed this way, you can expect another hour of daylight at least. Two hands two hours. Beyond this things get flaky because your hand is not exactly 15 degrees at this distance, and so on. You can use this to estimate in 15 min increments (of time, not degrees) by counting the number of fingers the sun is above the horizon as well.

 

This is all rough estimating, but if you are out looking for benchmarks, and want to make a quick check to see if there are obstructions above 15 degrees off the horizon, over a significant portion of the horizon, it is a lot easier, and faster to stand at the benchmark, hold your hand out, and turn through a complete circle than it is to try to find a protractor in the bottom of the trunk.

 

-Rusty