Land surveying... how does it work? How precise?

[filmore]

Sometimes I see what looks like a spider gear embedded in roadways, sidewalks or telephone poles. Are they related to surveying markers? Their placing looks deliberate and not just that they fell off a car and got stuck.

[Chronos]

Aside: I didn’t know gears like that had a name. Those are for connecting shafts at right angles to each other, right? Now I’ll know what to call those pieces next time I’m running a Lego robotics event.

[Channing_Idaho_Banks]

Anyone really interested in land surveying should check out this fun book:

The Great Arc: The Dramatic Tale of How India was Mapped and Everest was Named – August 22, 2000
by John Keay

The British Great Trigonometrical Survey of India was an epic undertaking spanning decades which took the measure of the sub-continent.
The book is a brief but lively biography of the two men who headed the survey - William Lambton and George Everest. The progress of their efforts across the Indian landscape makes for fascinating reading.
The amazing accomplishments of the Survey in the face of fever, tigers, and other resistance are highlighted in the book. The naming of Mount Everest is but an historical afterthought to the incredible saga of the Survey itself.The Great Arc: The Dramatic Tale of How India was Mapped and Everest was Named: Keay, John: 9780060195182: Amazon.com: Books

[74westy]

@whitetho

For the last 20 years, North and South Carolina have been working to come up with a more precise demarcation of their common border, scheduled to take effect on January 1, 2017. As part of the improved survey, it was determined that the Lake Wylie Mini Mart was actually in North Carolina.

A surveyor knocked on the door of an old farmhouse and spoke to the man who answered. “We’ve been resurveying the state boundary. It turns out your house isn’t in Vermont like you thought. It’s really in Massachusetts!”

“Good”, the farmer answered. “I don’t think I could take another Vermont winter.”

[newme]

@filmore

Sometimes I see what looks like a spider gear embedded in roadways, sidewalks or telephone poles. Are they related to surveying markers? Their placing looks deliberate and not just that they fell off a car and got stuck.

It is most likely a cotton gin spindle.

[Kron]

To answer questions about GPS:

We use GPS a LOT. The repeatable accuracy I get with a GPS unit sits usually around a couple hundredths of a foot horizontally, and somewhere around seven or eight hundredths vertically. Generally speaking, GPS is accurate enough (with good view of the sky) for most applications. If extremely accurate elevations are required, we will break out the more conventional instruments.

[filmore]

@newme

It is most likely a cotton gin spindle.

That’s them! Thanks. I guess they are used as survey markers.

[Jaguars]

@bump

…and when we came back to survey the finished house, the house was like 8 feet to the left of where they’d originally surveyed the form, and smack-dab in the middle of a series of easements drawn up for the neighborhood. As best as the guys back at the office eventually could find out, the construction guys decided that the form was inconvenient for actually pouring the foundations for that house and the house next to it, so they just shifted it over so they could back the concrete truck right up to it.

Story of my life! “Yeah, the platform was excavated too low so we took the floor down 200mm” Builds house below flood level

@Channing_Idaho_Banks

Anyone really interested in land surveying should check out this fun book:

Agreed, a very good book, and not, I think, too jargon-y for non surveyors.

@newme

It is most likely a cotton gin spindle.

Nice, perfect for hammering into a sealed roadway as a reference mark. Here in NZ we don’t have them so we have to buy specially made dome bolts.

[Isilder]

@Kron

To answer questions about GPS:

We use GPS a LOT. The repeatable accuracy I get with a GPS unit sits usually around a couple hundredths of a foot horizontally

And that is strongly the reason the OP has the problem.

The GPS is not consitently accurate because that variation in density of the atmosphere and that density change affects the speed of radio wave propogation to the extent of metres…

Sure you might find that 10 times you go to the one spot you get the same location down to fractions of a millimetre… but if the atmosphere happened to have deep lows inbetween you and a few satellites, it could possibility introduce an error, while still saying its got fine satellite contacts ! I suppose the chance of the atmospheric abhoration is quite small , since the disagreeing satellite will probably be ignored…But if these errors introduced by the atmosphere happen to make most of the satellites report pretty much the same error…

[Fir_na_tine]

When many parts of New England were being surveyed with rod and chain, inevitable errors were made when long stretches of forest were surveyed. When supposedly parallel lines didn’t meet after 10 or 20 miles, the difference is called a gore. See this link
Surveying gores

[Duckster]

You need to be careful about disturbing a zombie surveyor. They might not measure up since they are only out for blood.

[Marvin_the_Martian]

@Kron

To answer questions about GPS:

We use GPS a LOT. The repeatable accuracy I get with a GPS unit sits usually around a couple hundredths of a foot horizontally, and somewhere around seven or eight hundredths vertically. Generally speaking, GPS is accurate enough (with good view of the sky) for most applications. If extremely accurate elevations are required, we will break out the more conventional instruments.

I thought GPS for surveying requires two simultaneously operating GPS receivers, a base unit located at a benchmark (known location) and a roving unit which communicate with the base unit? Then GPS signal is used to calculate the difference between the base and the rover location, not the absolute position, thus canceling out much of the GPS error. I believe the OP is assuming a single handheld GPS receiver, which is a completely different animal…

[muldoonthief]

@Fir_na_tine

When many parts of New England were being surveyed with rod and chain, inevitable errors were made when long stretches of forest were surveyed. When supposedly parallel lines didn’t meet after 10 or 20 miles, the difference is called a gore. See this link
Surveying gores

I’d think parallel lines not meeting is a good thing.