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Friday’s LDP
The point of using an LDP for a small area is to simulate or be kinda close to “true north” since the original surveys are true north based. Also, as a throw away, the grid distances can be adjusted to be with a few PPM of ground distances. Of course the grid distances in state plane projections can be expanded by a factor and simulate ground distances but the bearings will be off however far from the central meridian your project is, in this case there would be a 2 degree rotation using state plane.
For this survey the origin point was placed onsite at a Lat and Long using the nearest minute. A minute of Latitude is approximately 6000′ so there was really one choice for each since the site is the W1/2 of a Section. Of course the Latitude could shift north or south and not change any bearing for the survey, the longitude is the critical choice.
Inversing between the SW and NE Section corners:
The grid and ground distances are close, the bearings are almost true north, which is what we are wanting.
The found south Section line:
The south line was fully recovered, it was original stone monuments, it’s very unusual that the grid bearings are identical after locating the monuments, replacing them with new caps and burying them alongside.
This is the South 1/4 which was recovered buried a foot in the ground.
The LDP is a Transverse Mercator Projection Scale Factor=1.000196
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3 Replies
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Can I see an example of how you document the LDP . Presumably you put notes on the drawing?
Also – I am curious why you didn’t use scale factor=1 at the meridian?
Thanks
Roger
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nm. re-read the op.
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A note on the plat explains the LDP origin point, the type of projection (Transverse Mercator-WGS84), and the scale factor; a scale factor of 1 would leave me 200ppm short for my grid distances compared to measured ground distances, or about a foot per mile of “error”. I don’t give anyone a false northing and easting since I’m not publishing XY coordinates, any XY values will work attached to the origin point. For this job I threw 100000, 100000 on the origin point.
The program allows a report to be printed out with all the data, I place it in the job folder, it’s one of the sheets of paper that gets trashed when the job folder is scanned.
Geodetic north and Grid north will mesh along the longitude of the origin point, otherwise there will be a small convergence angle east and west from that point through the project, but it’s a small project and in 1880 the ability to measure a true north bearing wasn’t so refined, these grid bearings today should be considered a decent way to begin a retracement for the true north bearings of 1880. If you can read my second picture (sorry it didn’t show up very well) you can see I’m within 1’30” of their measurements which says a lot about those old surveyors. They knew what they were doing!!!
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