@field-dog ok so grid state plane coords are just plane coordinates simple math. Not necessarily geodetic. Only time a extra step comes in for computations is for ground to grid distance or grid to ground distance . Unless you want to get into the weeds and start considering things like deflection of the vertical etc. grid bearings are just the simple math. The convergence angle is to get from grid bearings to geodetic etc. north. True north is equal to astronomical north or some geodetic north. The lines of longitude on a handy desk globe converge at the poles so they are nat parallel like grid bearings. Now i am not in a plss state so i think ask the experts on the 1/4 and section corners. It was prior to the year 2000 when i did sections. If i remember correctly most of those were done via astronomical north. You can still get from grid to geodetic via convergence then geodetic to astronomical via laplace correlation if needed. On very long lines t-T
This is the sort of stuff I like. I was a little disappointed I don't get to do much of this in the working world since everything is computerized.
In fact, I think the closest I came was when a contractor asked for stakes for a manhole that landed in the middle of an intersection that was tore up and being worked on in all 4 directions. I was staking everything by station and offset but he asked if I could put them in the adjoiner's yard which mean they would be at about a 45 degree angle to the manhole. I did some math in the truck to calc a couple points and made it work. As a recent graduate in particular that was a cool thing to be able to do.
@field-dog you keep learning. Thats what it is all about. This site is great for this. We never stop learning and as soon as we think we have it all figured out the non flat earth humbles us to learn something knew. A new publication will be released soon nos 92. I believe. I believe they stated at FIG that a alpha version would be released. But I didn’t take good notes unfortunately. I was to busy wrangling kids at pool. But the NGS day had a lot of great information so lots of things going on that will be out for us all to learn.
@field-dog hey on NGS site there are numerous videos presentations on datums coordinates systems etc along with all the publications. It cost you nothing and truly is a great resource. Some you can even take a test and get a certificate for. Some are short some longer. But all of that information is available for the knowledge thirsty folks. When i was coming up i could only read. The different publications and one of my all time favorites was geodesy for the layman. Now you can pop popcorn and watch on a smartphone lol. Just keep learning. The profession needs people just like you who are constantly learning. And wanting to learn. I learn something new all the time. But i am not the brightest pea in the patch. But it’s so much fun. Keep it up.
I believe they stated at FIG that a alpha version would be released.
I wanted to attend some of the classes at FIG, but it was too expensive. Were Carlson and Leica there?
@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.
