Terminology Assistance Please...

[a/r/sf/ret]

Okay you surveyor guys and knowledgeable benchmark hunters...clue me in as to the definition of "grid feet" and "grid bearing".....if my training was right, I understand those terms were measured from a map or on the map. Not sure though. Help please.

[Z15]

Grid: Two sets of parallel lines intersecting at right angles and forming squares; the grid is superimposed on maps, charts, and other similar representations of the Earth's surface in an accurate and consistent manner to permit identification of ground locations with respect to other locations and the computation of direction and distance to other points. 2. A term used in giving the location of a geographic point by grid coordinates.

 

 

Grid North: the direction of a grid line which is parallel to the central meridian on the National Grid.

 

True North: the direction of a meridian of longitude which converges on the North Pole.

 

Magnetic North: the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate.

 

Grid Bearing: Bearing measured from grid north.

[+Kewaneh & Shark]

The term 'grid measurement' is a geodetic term that is generally used when defining measurements that are taken from a map. The term is used so that the measurement is not confused with a measurement taken on the ground, aptly termed 'ground measurement'. Because a map is a flat plane and the ground (earth) is a spheroid, the measurements from one to another are not equal without a mathmatical conversion formula. The formula, or conversion factor depends on the type of mapping projection that is being used in the local area (Which is usually defined by State law).

 

Small survey projects usually don't deal with any type of conversion from grid to ground measurements. The project is assumed flat. However larger projects, such as large geodetic control networks and surveys, highway construction, hydro-electric projects, or other survey and engineering tasks that cover many miles of a curved earth must be represented accurately on a flat map. The 'ground' measurements are converted to 'grid' measurements for mapping purposes. A conversion factor to convert the numbers back to 'ground' numbers should be shown on the map.

 

While the question and answer of 'What is a grid distance?' is relatively simple, the 'why is grid distance' is anything but simple. It is based in spherical trigonometry, mathmaticians and mapmakers have been working on it for centuries, and it can be 6-10 units of study at a college with a Survey Engineering or Geomatics degree.

 

- Kewaneh

[evenfall]

Keep in mind too that the Earth is round and perfect squares do not necessarily describe something round really well, so grid north does not exactly point true, except for on it's base meridian. Where is that Base meridian? Well, it is not where you might think... It comes from a different way of looking at the world. A way that does not take into effect the curvature of the earth as well as we would like, but a way that works if we keep that in mind when we use it.

 

On the grid based idea, the State Plane Coordinate System was born. It is a form of Lambert Conformal Conic projection, only it was given an accurized baseline for each state, Rather than 2 edges of a world map which is the usual manner with Lambert. It was a way that a flat map could fit a territory, even if the way we thought of it didn't hold true for every construct about the territory that there is. Of course anywhere off the baselines is a a geodetic distortion, but it helps describe things accurately, or well in a measurably accurate way if you don't account for everything, and locally when compared to that baseline.

 

NGS Developed the SPCS back in the NAD27 days and there used to be this thing that came out of it called the US Survey foot that was part of it. The Grid foot, or U.S. survey foot = 0.3048006096012192 meters, exactly. We as a country no longer use it. 1986 came along and so did NAD83, NGS went 100% metric, dropping the old survey foot. Now you convert Meters into international foot with the Formula: Feet= Meters/0.3048 exactly.

 

Read more here: http://www.vterrain.org/Projections/sp_feet.html Follow the links you find there too if you like.

 

There are your Grid Feet. Oh and the Grid they are working with too. For most of us, it will be helpful not to put too much energy into it. It is just another mind numbing construct, albeit needed to fulfill some purposes, like using a grid system to describe something instead of Lat/Lon via Trig, in the survey world. Like UTM only different.

 

Enjoy,

 

Rob

[DaveD]

evenfall - 25 states have legislatively adopted the U.S. Survey Foot for conversion of their NAD 83 state plane coordinates while only 8 have adopted the international foot, the other 17 have nothing defined by legislation. The problem of which foot conversion to use is a constant source of misunderstanding to many surveyors and GIS professionals.

[a/r/sf/ret]

Thanks folks....learning more as i go.

[+BuckBrooke]

DaveD, which states are which?

[evenfall]

Hi Dave,

 

<grinning> Yeah I had heard... I even know of a county where they are very, no, make that VERY adamant that all vertical control be done in NGVD29 to this day. It is a pain.

 

Politics and bureaucracy as usual. Makes one wonder how we update anything when half the world insists on staying in the past. It goes to show that the old saying is true. All politics are local. Yup!

 

Too Funny!

 

For us here at geocaching the international foot will work well enough.

 

Rob

[Bill93]

Is it standard practice to take the SPCS, which was defined in meters, and work with it in the field as feet?

 

I recently looked at some construction survey stakes for road work near my house and was trying to make sense of their numbers. I now have found the NGS utility for converting to SPC, and my lat-lng values converted compare very nicely to what is written on the stakes. One is within 3 feet and the other is 10 feet off. The distance I get from my handheld readings is 1411.5 ft and the hypotenuse from the numbers on their stakes is 1400.091 ft. Close enough for handhelds.

 

I would like to be able to read in this system on my handheld Garmin 76s, but I don't find the right options in its list. I faked out my handheld's User UTM system so I can get it to read pretty close in meters to the numbers on the stakes converted to meters. I can't find any option for Lambert, or even Mercator passing E-W through an origin, so I used the UTM on standard meridian with the SPCS origin and SPCS false easting, and a huge negative value for false northing to make it agree. Obviously this won't track well over the state because the base line is the wrong direction as well as being Mercator when it needs to be Lambert. Is there anything else I can do with a Garmin 76S to make it more accurate for SPCS?

Edited April 27, 2005 by Bill93

[evenfall]

Bill,

 

Yes it is common to convert things to feet because Americans Think in feet and use feet.

 

Construction Staking is no exception. Everything is planned to feet. Meters are sometimes used on highways, but that is not always the case. Many States are dropping the requirement for it as too many things go wrong. American's make mistakes with measuring systems they do not understand but it goes beyond this. They are actually resistant to learning it. Rather than fight them on it, it is just easier to convert. Further there are no real tidy metric numbers that fit what America has already built in feet. All the conversions to metric were more crazy making than converting metric to feet.

 

The reason you see metric system usage in NGS is because of Geodesy. The meter itself is a derivative of geodetic measurement. It takes ten million meters to go from the north pole to the equator. It was a given distance and the French Academy of Sciences decided that the appropriate sized unit would be that distance divided by ten million. Since the meter was designed to describe geodetic measurements, it still is.

 

In construction Staking, we so not add the SPC info on the stakes as it is not what a constructor needs to do the job. It is written in codes, but it basically tells them what to do, how far away from and what quantity from where we place our staking. Decoded, it might say something like "25 feet away from here on line with these two stakes, is the location of the Top Back of Curb. The Cut to Top Back of curb is 3.07 feet to Finished Grade. It can be a lot more elaborate than that but that is the side of the stick facing the work. On the back of the stick I'll write the Job point number and the elevation for the Hub, with or without Tack that I drove in the ground in front of the Lath I wrote the directions on. In the GIS the SPC of that spot can be derived, so can the Lat / Lon, but it is not important to the constructor who's main gig is to move dirt and get paid. Construction Staking is a ways down the food chain from Geodetic Control, but the construction staking could not be meaningful without the Geodetic work in place.

 

In summary, the Metric system is a good fit for Geodesy, but a difficult fit for the American way of thinking.

 

Rob

Edited April 27, 2005 by evenfall

[DaveD]

U.S. Survey Feet - California, Colorado, Connecticut, Delaware, Florida, Georgia, Kentucky, Illinois, Louisiana, Maryland, Massachusetts, Mississippi, New Hampshire, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas, Virginia, Washington, Wisconsin.

 

International Feet - Arizona, Arkansas, Michigan, Montana, North Dakota, Oregon, South Carloina, Utah.

[+BuckBrooke]

The meter isn't defined geodesically anymore, and so has lost some connection to surveying.

[ArtMan]

The meter was the last basic unit of measurement to be defined in terms of a physical thing - a metal bar kept at the International Bureau of Weights and Measures in Paris. Now it is measured in terms of the speed of light. For more information see the Nat'l Institute of Standards and Technology (NIST) website.

 

-artMan-

[holograph]

Warning! Topic drift!

 

I think the kilogram has the honor of being the last unit of measure still defined by a physical prototype. There are a couple of proposals to redefine mass in terms of fundamental constants of nature like all the other units of measure, but they are not yet ready for practical use.

[evenfall]

Buck,

 

I would not go as far as to say what you said, and I am not sure you can substantiate it. I'd be happy to let you try if you like though! :-D

 

The meter has it's history in the physical length as a line from the Equator to the North Pole as taken through Paris. it was first defined as 1/ten millionth of that measurement. That is a Geodetic Measurement. Science needed a more stable object than the earth and a physical unit for comparison so they made one several times and adjusted it a bit because of further observations.

 

The meter today:

 

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. It remains the main unit of geodetic measurement today.

 

While it is true that the meter is now defined differently, it has not lost it's relationship to the work of Surveying.

 

Yes, the Surveyors and engineers DO rely on it quite heavily. Less so in America, but is underlies a lot of it. We just use the US Measurements because it works best for the US thinker's frame of reference. That is a cultural resistance thing more than anything else. A lot of us Survey is done and Has been done in the US standard of feet with no connection to the Standards of Geodesy too. After the fact it is a bit of work to make it all fit well too. I will reiterate that all NGS Control is based on the Meter, In fact this is true of Geodesy in every country in the world. So all Geodetic control issued in the US is based on the meter because the NGS is the purveyor of that control. The math is done metrically and converted to US standards after the fact in most cases. It is the way Geodesy has always been done. And that is the most accurate Horizontal or vertical control we have, period. There is no other Higher order for it in America, including the work done by NGA, the WGS Datum Folks. They use the metric system too.

 

Holo, Maybe we need outriggers or stabilizers like they use on ships to help with the topic drift eh? :-D

 

Rob

Edited April 27, 2005 by evenfall