topic Re: Creating a Local Projection - ArcMap V10.2 in Coordinate Reference Systems Questions

Creating a Local Projection - ArcMap V10.2

timdunlevie — Thu, 30 Oct 2014 06:34:50 GMT

Hi all,

I'm sure this has been asked before....

I have grid parameters of a local projection (Mine grid) based on the Map Grid of Australia:

Grid Name : Local Grid

Central Scale Factor 1.000000

Origin Local Grid Easting 800,400.025

Origin Local Grid Northing 300,500

Origin MGA Zone 50 Easting (GDA94) 700,000.498m

Origin MGA Zone 50 Northing (GDA94) 6,900,600.233m

Orientation at Origin 31o24’43”MGA

Now to add a new grid these are the steps I am using:

1. Data Frame Properties

2. Coordinate system tab

3. New Projected coordinate system

4. Change Projection name from Transverse Mercator to "Local"

5. Enter the above parameters into the "Values" area, where:

False Easting = Origin Local Grid Easting 800,400.025

False Northing = Origin Local Grid Northing 300,500

Scale Factor = 1.0000000

Azimuth = Orientation at Origin 31o24’43”MGA, but I put in decimal degrees of 31.411944

Longitude of center = Origin MGA Zone 50 Easting (GDA94) 700,000.498m - converted to Geographic coords and to decimal degrees (GDA94)

Latitude of center = Origin MGA Zone 50 Northing (GDA94) 6,900,600.233m - converted to Geographic coords and to decimal degrees (GDA94)

Unit = meter

Meters per unit = 1

Geographic Coordinate system = I changed this to the MGA Zone 50 (GDA94) grid...which the local grid is based on.

I don't seem to get real accurate results.

The points are a fair way off where they should be.

Have I done everything correct ?

Thanks,

Tim

Re: Creating a Local Projection - ArcMap V10.2

MelitaKennedy — Thu, 30 Oct 2014 16:17:48 GMT

Hi Tim,

Are you seeing a rotation? The azimuth value you have might be relative to the MGA zone rather than geodetic north, which is what the Local projection uses. You might see if you can find the grid convergence at that location and then adjust the given azimuth.

As long as the mine data still has a undefined coordinate system, you can keep modifying the data frame's definition to try to better the fix. I usually take definitions like this as a start point, but then spend a bit of time, improving the definition.

Melita

Re: Creating a Local Projection - ArcMap V10.2

timdunlevie — Thu, 30 Oct 2014 23:58:20 GMT

Ahh good point!

Will give that a go.

I did also alter false easting & northings which made the data more “accurate”…but was trying not to fiddle too much with the parameters I was given.

Many thanks,

Tim

Re: Creating a Local Projection - ArcMap V10.2

timdunlevie — Fri, 31 Oct 2014 05:36:54 GMT

righto...updated the angle of rotation to match the angle & grid convergence....this was a much better fit.

I then altered the angle slightly to match as well as required (+/-1m).

so all good.

Many thanks,

Tim