Do all grids not align to true north?

ƒ??Texas North Central 4202" is the State Plane Coordinate system, which has the job of mapping the round earth on a flat plane.?ÿ Something has to give, and one of the things here is that portions of the plane map will have the lines point in slightly different directions.?ÿ There is a correction available for any given location to find how it differs from geodetic north.

Can you set the rover to display latitude instead?

Grid north is true north only at the origin - based on the grid definition?ÿ

As you move on the plane away from the origin - north is held parallel, so you need the convergence angle ( how far east or west of the origin you are ) to adjust back to "True North" - If you use A Longitude reading - the display needs to have at least 3?ÿ decimals of a second ( one second of arc is many times more than 100 feet )

I did a project to align a new solar array, so I measure a few control points - then reduced the data in starnet and had the Convergence angle for each point and was able do the layout with my total station - they checked it with solar machine they had ( cannot remember what they called it) and liked the results

of course if you are they about noon, you could take a solar shot?ÿ

To get true north would require the use of geodetic bearing and not a grid bearings. If using a State Plane projection, you would need to apply the mapping angle at your specific location within the grid projection to correct for convergence to get you to 'true' north. If you were on the central meridian of the grid projection, the grid and true north would match, however the further you move east or west towards the edges of your state plane projection, the correction for convergence will steadily grow (as much as a couple of degrees). So short answer is in state plane your eastings will remain the same, however the lat/long (geodetic) will change with the convergence angle. Sort of like trying to wrap a flat sheet of paper (grid) around a ball (geodetic), without folding it.

This new technology is great, they said.

It's so simple to use, they said.

People don't need any training or experience to use it, they said.

Lines of longitude converge as you run north, coming to a common point at the pole.

North everywhere on a State Plane Grid System runs parallel to the central meridan of the system (ie/one specific line of longitude) .

So if you are west of the central meridian of a grid system and tracing a line of eastings as you move north, the geodetic line of longitude that you started at is progressively drifting off to the east of your track.?ÿ ?ÿIf you are east of the C.M., the opposite occurs.?ÿ The line of longitude you started on?ÿ drifts off to the west as you continue north.

This is what the convergence angles?ÿ are all about.

Posted by: @rover83

People don't need any training or experience to use it, they said.

Lets give the OP credit for recognizing something was going on and exploring for answers.

Posted by: @rover83

This new technology is great, they said.

It's so simple to use, they said.

People don't need any training or experience to use it, they said.

I think "they" may have said that about the Atomic Bomb too!

Posted by: @xuv224

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I used a Trimble R12 Rover over an RTK network on land in North Texas a few days ago. The goal was to stake out a line pointing to true north (and another line going due east-west).

The collected data was in the state grid ƒ??Texas North Central 4202ƒ?, formatted in Easting and Northing, in US survey feet.

The points were staked out along the north-south line, starting with a reference point, then further points on the 180 degree line south of there.

When these points (Eastings and Northings) are then converted into Lat Long on a site like https://www.earthpoint.us/StatePlane.aspx they come up with a different longitude for all given points, even if the exact same Easting is used to input into the converter.

If the Easting number of each point stays exactly the same and only the Northing is changed in the above conversion website, should the resulting longitude not also stay exactly the same?

Thanks,

Marco

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I think if you put the lat/long of one of your points into the NCAT tool on the NGS website itƒ??ll give you the convergence angle for your point. If you set up on the point and backsight another to the north you turn the convergence angle to geodetic north (right if negative, left if positive).

The simple way to do this is to input two points into Trimble using the identical longitude with different latitudes. Then stake along that line, checking longitude of the staked points.Trimble makes this so simple.

Posted by: @mightymoe

The simple way to do this is to input two points into Trimble using the identical longitude with different latitudes. Then stake along that line, checking longitude of the staked points.Trimble makes this so simple.

Also, understanding geodesy and map projections help a little.

Here is a simplified diagram of a Lambert map of the form of the Texas North Central SPCS:

The black circles are latitudes, the orange lines are longitudes, and the background Cartesian coordinates are the map grid. The orange lines converge at the pole, the point (0, 25) in this case, so they point "due north." The key point is that latitudes are curved lines, longitudes do not have infinite slope, and the mapping grid is composed of straight horizontal and vertical lines that mark "grid north" and "grid east."

The picture below zooms in on a portion of the map that shows a blue line of constant Easting. If the initial point is P4, then the line would have to be rotated to the orange line that contains P4 in order to run true north. After this rotation, the Eastings would no longer be constant.

The issue is that we're dealing with two different coordinate systems. There is a one-to-one correspondence between the coordinates of individual points; ie, each point has both a unique lat/lon and a unique northing/easting, so conversions between coordinates are routine.

Thus, we can have a line of constant longitude or one of constant Easting, but, except for lines on the central meridian, we can't have both at the same time. Reconciling the two takes a bit more math, but the further the line from the central meridian, the worse the problem.

We live in a great time where learning can proceed from observation to theory much more conveniently than in the old days. Those old guys who could both visualize and figure were amazing,

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@edward-reading?ÿ

For this problem I'm going to make some assumptions. R12 implies that a Trimble DC is involved. You can enter a point and Trimble has a neon sign and flashing lights saying enter me geodetically, enter me geodetically. You want a true geodetic north line, for heavens sake enter it geodetically. First principals, simple is easier. The coordinate projection is totally irrelevant, it will stake the same line regardless of the projection. Understanding projections is important if you're going to work in them, but the OP wants something else.

In my neck of the woods, the State Plane version of North is close to two degrees off from Geodetic North.?ÿ We see this frequently on surveys from different decades.?ÿ The 1990 bearings disagree with the 2010 bearings by roughly that much.?ÿ I'm sure those lay people looking at plats showing both bearings as record and measured wonder what kind of crazy people surveyors are.

This Jan van Sickle article should be read by OP for further understanding.

@bill93

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Thank you Bill,

You mentioned that there is a correction available for any given location. How do I find that correction method?

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The primary coordinate readings were these:

North Point

X 2679903.597

Y 7132785.259?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Z 642.524?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

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South Point

X 2679903.599?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Y 7132414.341?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Z 636.980?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

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This south point was staked out on the 180?ø azimuth line, using the rover.

But when inputting the South Point coordinate into the conversion tool to Lat / Long (found at EarthPoint), the resulting point shows more west than it should be.

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East Point

X 2680135.016?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Y 7132586.545?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Z 645.042?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

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West Point

X 2679722.592?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Y 7132586.520?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

Z 637.426?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ

This West Point was staked out on the 270?ø azimuth line, using the rover.

But when inputting the West Point coordinate into the conversion tool to Lat / Long (found at EarthPoint), the resulting point shows more north than it should be.

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I attached an image with these four coordinates imported into Google Earth and they do not align as was expected.

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There were also another 20 points staked on the land and a checkered drone GCP target flag placed over those points.

The drone software needs Lat Long data of those points, so that it can correct the final aerial image to be precisely aligned with the correct Lat Long.

But if all of these readings cannot be converted into Lat Long and be correct, than that would introduce more errors than just letting the software use the GPS data from the drone (which is not very precise, but probably still more precise than converting the (very precise) Trimble readings into a false Lat Long.

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Thanks,

Marco

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Stake out to the North Pole. Simple.

@xuv224 an east-west line is curved.

Mighty Moe had the best solution.

get the lat/long of the intersection point (I assume the building shown is proposed, not in existence yet).

Stake the north point and the south point on the same longitude as the intersection point.

Stake the east point and the west point on the same latitude as the intersection point. Note if your equipment is perfect then your 3 points wonƒ??t line up but in your short distance this is unlikely to be noticeable.

@xuv224?ÿ

You don't need to fight this. Use the Lat Long values holding one long. The resulting grid AZ between them is the convergence angle subtracted from grid north.

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Actually if you use some of the routines on your DC you can sit in the field and observe the longitude and latitude. I don't know what type of DC you have or what the programs you have, but with Trimble this is super-easy.