Multiple coordinate systems due to local grid conversions

[Anonymous]

Hi there,

I'm looking for advice on working with multiple coordinate systems between surveys. We are working in local grid, which is split in to two sections (two local grids). These both have different conversion factors from National Grid (Move and Scale) and unfortunately don't tie in until you convert them back to National Grid so we have been designing in MX in two seperate sections.

What would be the workaround for this to incorporate the two surveys/coordinate systems in to one drawing?

[Pointdump]

Matt,

Why can't you just keep everything in OSGB? Or a UTM? I'm not familiar with "Zones 6 & 7", but it looks like you could make a custom Oblique TM coordinate system that would work from end to end. The nice thing about custom projections is that you can still use Bing Live Maps.

More information please on what you need to do with your drawings. Survey Plats? GIS? Corridor Design?

Dave

[Anonymous]

Zones 6 &7 are simply the survey company's labels...

The design has all been done in MX in local grid (in 2 seperate models) so it would be a conversion - it's a 30km+ corridor design.

Due to the curvature of the earth over a long distance we don't use National Grid and the conversion factors mean they don't tie up in Local Grid.

[Pointdump]

Matt,

A custom projection would be ideal, except for herding all the cats involved in the project, using who-knows-what software. Many of the State DOT's here in the Colonies use custom projections for zone-spanning corridor projects.

Where in the UK is your project? Can you use UTM-30N?
http://whatutmzoneamiin.blogspot.com/p/map.html

Dave

[Anonymous]

I've attached the rough area of our project.

Admittedly i don't know enough (next to nothing) about surveys and surveying to comment on a custom projection system which is why I came looking on here for adivce. Bit of a head scratcher for me this.

Thanks for your assistance so far.

[Pointdump]

Matt,

A "stock" Coordinate System would be my first choice, before creating a custom one. This one looks promising: ETRS89.UTM-30N. It should cover your project pretty well.

 

Dave

[Pointdump]

Matt,

Because British National Grid is a Transverse Mercator, it should work for your project area, and it seems to be the prevelant CS where you are. There's several UK members here. I'm hoping one of them will chime in.

http://geo.nls.uk/urbhist/guides_coordinates.html

Dave

[Anonymous]

Having spoken to the Geomatics team, it has been confirmed that we cannot use another coordinate system. More information on local grids can be found here: http://www.standardsforhighways.co.uk/ians/pdfs/ian99.pdf Apparently going east to west, the maximum you can have is around 15km before measurements become incorrect.

Unfortunately that still leaves me with the problem of having to use two coordinate systems in one model - as you say, hoping some of the UK users can pipe in here.

I believe there is something called "Snakegrid" that adds a "twist", that some users in the rail teams use but that's not compatible with autodesk products yet.

[Pointdump]

Matt,

"...the maximum you can have is around 15km before measurements become incorrect"

The earth is round and AutoCAD is flat. Everything in AutoCAD is done on a projected FLAT surface. ALL Map Projections contain distortions, both for Scale and Elevation. These distortions are NOT errors; they are calculable and easily dealt with. Your choice of Map Projection will depend on what data you're using (Boundary and Topo Survey) and who you're going to hand your design to (Construction Staking Surveyors).

There is absolutely no reason you can't use British National Grid for the entire project, from start to finish.

Dave