Local System - Underlying Coordinate System

[Shawn Billings]

Or perhaps you are near the central meridian and the coordinates were rotated to geodetic...

 

[John Thompson]

I'd be curious to see where that much rotation came from.

It came from not rotating the deed polyline to the Google Earth image. I knew some of those corners would be at fence corners and easy to find, so I didn't bother lining everything up beforehand.

 

[Jim Frame]

The Users Guide, page 112, says "It is critical that points are imported into a new Local coordinate system if they are to be used to setup a new localization." Does this mean that they can't be imported first and the coordinate system then changed to a new local system? (Or is that even possible?)

 

[Matt Johnson]

It is not possible to change the coordinate system of design points. They become associated with the coordinate system they are created in at the time they are created.

 

[Jim Frame]

It is not possible to change the coordinate system of design points. They become associated with the coordinate system they are created in at the time they are created.

Is that a bug or a feature?

 

[Matt Johnson]

It is a feature. Design points can be displayed in another coordinate system but the transformation from their local system to WGS84 is always defined by the coordinate system they are imported into.

 

[Phillip Lancaster]

Explain to me why on earth you are using 5000, 5000 when you have an LS? Your clients like a small coordinate system? Coordinates are coordinates. SPC on the grid are the only way to go. It is stupid simple. 5000, 5000 vs SPC = Hard vs Easy. I use to do local until a light bulb went off (ten years ago) and I have never look back. It would require more work and headache to use 5000, 5000 again. I just don't get it.

 

[Jim Frame]

There are many situations in which non-SPC coordinates are useful, but if your general question pertains to localization, for me it's a search tool. I often have the dimensions of a boundary, but the corners, distances and directions have no known relationship to the SPC grid. For purposes of localization it makes little difference if I calc those corners from an estimated (e.g. Google Earth) SPC-like position or from 15000,15000. Localization will make the connection between my search coordinates and the SPC grid for search purposes.

Going back to the reasons for using non-SPC: most of the boundaries I work on are referenced to something other than grid, typically a bearing base defined by monuments in the ground and a document of record. So I'll adjust my survey in Star*Net, and if the survey is on grid (much of my work doesn't involve GPS measurements at all) I'll instruct Star*Net to create a ground-scale points file that matches that bearing base. Whenever I do that, I make sure that those ground-scale coordinates are radically different from SPC (e.g. 15000,15000) so that they can't be mistaken for same.

The other common situation in which I use non-SPC coordinates is when I'm working on an existing project that's on a non-SPC grid (or on an older SPC epoch), in which case I'll match my work to the project grid.

 

[Phillip Lancaster]

I get that Jim. Old jobs being 5000, 5000 would be the "only" reason for me. But if you forget about old jobs and had to choose between a local or SPC for a new job. Which would you choose. Lets just say hypothetically if there was an "LS" back in the 80's, 70's, or 1800's that works just like today. Would you choose to work in 5000, 5000 or work on a grid. How many surveyors would jump at SPC's? I choose this because of the growing database of all my jobs being tied together. Creating a network of boundary corners or previous locations of those corners that are still good decades down the road. The extra work and it is extra work in using a local system doesn't make any sense to me. Its like saying. I only care about this one job and this one job only. Of course I cannot live without Google Earth so SPC's for me is the only way to go.

 

[Jim Frame]

Creating a network of boundary corners or previous locations of those corners that are still good decades down the road.

Where I am our corners (along with everything else) are moving northwest at about 2 cm per year, so that decades thing doesn't work out very well around here.