Survey Coordinate Conversions

Hi Keith,

If you only want to convert between UTM and your local coordinate system you can use Survey | Convert Coordinates Between | Plane Grids.

Setting up your conversion parameters is pretty simple and you can do it in two different ways, depending on the available data:

• Using one point whose coordinates you know in both coordinate systems, along with an azimuth (bearing) correction and scale factor
• Using two points whose coordinates you know in both systems. This is the more popular method.

Presumably you already know the local and WGS 84 or NAD 27 UTM coordinates of some locations, and that's all you need to set up a conversion, which is a simple affine transformation. (Although UTM is a "proper" coordinate system, Micromine doesn't need to know it for a plane grid conversion.)

With respect to the other conversions on the Survey menu:

Convert Coordinates Between | Coordinate Systems is used for converting between different coordinate systems used around the world, for example between UTM on NAD 27 and geographic on NAD 83.

We've provided a bunch of example form sets to show you how it works, and if your desired conversion isn't in the list then you can define one yourself. A full coordinate system definition needs three pieces of information:

• The datum, which comprises
  • The transformation between that datum and WGS 84 (effectively moving the "centre of the earth", and optionally realigning the polar axis, from one to the other)
  • The shape of the ellipsoid used to approximate the earth's surface

• The projection, which defines the mathematical shape of the coordinate system on the ellipsoid.

In Micromine you define the projection on the Coordinate System dialog and then drill into the Datum dialog to define (or select) the datum and ellipsoid. I'm guessing you know this stuff already, but if you're hunting for the underlying parameters I'd recommend the GeoRepository website.

It's perfectly okay to use an existing coordinate system as a template. For example, you could define a NAD 27 / UTM Zone 11 coordinate system by opening the existing WGS 84 / Zone 11 form set, changing the datum to NAD 27, and then saving the coordinate system dialog as a new form set. You must save it as a new form set before clicking OK or you'll overwrite the original version.

You can add a local conversion to this process, but it's really only necessary for things like GPS data, where you might want to convert directly from WGS 84 lats/longs to your local coordinate system. (The opposite conversion is great for uploading planned locations to a GPS.) The only prerequisite here is that you need to know the locations of two points in a "proper" coordinate system along with your local system. In other words it's just like the Plane Grids conversion, with the only difference being that Micromine needs to know which "proper" coordinate system to use.

To enable a local conversion, set the projection to LOCAL, enter the local and tie point coordinate values on the Plane tab, and then choose the tie point coordinate system from the form set list. For obvious reasons the tie point coordinate system can't be geographic, geocentric, or local.

For now Micromine is limited to 3-parameter and 7-parameter transformations and doesn't support distortion grids. If you need to do a high precision conversion you'll have to use a GIS or geodetic software.

The Geographic and (U)TM and Datum (Molodensky) conversions are older and are essentially superceded by the Coordinate Systems conversion.

I hope this clarifies things.

Frank