@field-dog I was there to support the wife and wrangle kids. I know trimble javad hexagon etc. I imagine carlson and others were there also. But I didn’t go to any sessions except the NGS day one. I was sick the week before and just didn’t register and the kids wanted to play in the pool lol. Maybe next time i will do more sessions. But it was truly last minute for us even going. The wife decided that it would be fun for kids. I was lucky to get to attend ngs day. We got a babysitter that day. Well the wife of a co worker said i know you want to attend some sessions so she watch the girls that day.
@rover83 Wow! That publication brings back memories from the early 90's, having to study and learn both 27 and 83 for LS state exam. I have forgotten more than I have know :). Jp
ok so grid state plane coords are just plane coordinates simple math.
Thanks for clarifying that. If I understand you correctly, I would use a convergence angle to orientate my section lines to true (astronomical) north. I've attached a plat of a project I'm working on so you can see how the center of section line relates to the plat. I assume the north arrow is pointing to grid north.
Adjusting the grid azimuth by the convergence angle produces the Geodetic Azimuth. Adjusting the Geodetic Azimuth by the LaPlace Correction produces the Astronomical Azimuth. For point AK7150, which is within a mile of 48451, the LaPlace Correction is -1.70 seconds, likely insignificant, but necessary to keep the nomenclature consistent.
The picture below is from NGS INVERSE, calculating the ellipsoid distance and geodetic azimuth between 47897 and 48451. Note that the ellipsoid distance converts to 5279.037 feet, a thousandth of a foot different from the State Plane calculation, and the geodetic azimuth is about 1/4 of a second different from the geodetic azimuth calculated from State Plane.
@field-dog you would use the convergence angle to go from grid bearing to geodetic bearing based on that datum. Then use the laplace correction to get to true or astronomic north. Slightly different depending. If my memory serves me in the plss makes sure of the direction they ran in to compute as true north is not a simple 180 degrees difference forward and backwards. Thats probably in the weeds but if you want to learn then learn it correctly. As then you don’t have to assume when something is close enough. So let’s look at this. A plat has a magnetic bearing from say 1960. Pre nad 83. Can we use that magnetic azimuth or bearing to get to grid bearing. In theory we can. We can look at the magnetic declination for that time and convert to true aka astronomical north then we can apply the laplace to get to geodetic then project it to the grid so in theory we can go from one to another. Now in reality we don’t go through all of that always. We simply look at monuments held hold a line and we use say interior angles from bearings etc. if you look at old maps not plats they have a declination for that date between grid and true and magnetic. We can even look at the magnetic azimuth from say 1960 and see the difference of today’s magnetic and apply the difference. But we also need to realize how they might have done or performed a magnetic bearing and its accuracy. A compass so not to the second. It is why angles and distances fall down on the list of priority of calls. We use the bearing and distance to check. But the natural monument holds above the rebar pipe and is hen measurements. And measurements hold above the area etc. one thing that’s awesome about surveying is you cannot ever stop learning. Always something to learn.
north is not a simple 180 degrees difference forward and backwards.
Geodetic north and south are exactly 180 from each other. If there is an east-west component to the line then forward and reverse will be slightly different from 180 apart. Probably negligible on lot surveys but quite significant for a long E-W corridor.