Help with local grid projection

[Tom & Lisa Orr]

Hi All

 

I need some help with projection files.

 

I need a projection for a local grid. We have a number of points in both the local grid and AMG Zone 55 (ADG66), two of which are

Local 10548.4E, 9351.14N is at 322640.51E,  8076544.46N

Local 9882.12E, 10243.43N is at 321966.40E, 8077430.97N

 

How can I use these to create a local projection file?

I realize that I must edit the  

"AMG Zone 55 (AGD 66)", 8, 12, 7, 147, 0, 0.9996, 500000, 10000000

projection but not sure how to go about it?  I have made MapInfo projection files in my distant past and cannot really remember how it was done.

 

Up until now we have used the Encoms Discover add-on to transform vector files from the local projection to AMG but we would like the raster images to display correctly as well. 

Any help would be appreciated.

 

Regards

Lisa Orr

 

Orr and Associates

4 Wildsoet Street

Wongaling Beach

QLD 4852

 

 

[Spencer Simpson]

You want to apply an "affine transformation" to that projection.  Appendix H of the MapInfo Reference describes how to do this.  In the 8.5 version, the section on editing MAPINFOW.PRJ is on pages 657-662, with the part about adding an affine projection on page 659.

 

But before you can do that, you need at least three points to calculate an affine transformation.   I don't know of a separate tool to take control points, do the required linear regression, and calculate the affine transformation for you.   

 

You might be able to get away with creating a raster image that has enough pixels to represent your 'local' coordinates, using MapInfo's raster registration to match up the control points, then copying the affine transformation written into the .TAB file for the registered image.

 

Hope this helps

Spencer

 

 

---

[Steve King]

Lisa

 

You can also do this in Discover.  Set up a control file with your known points (the manual tells you the correct format) and then use them to transform a dataset.  At the same time you select the option to generate the projection clause.  When you do this it will appear in notepad.  Just copy and paste into the appropriate place in the Mapinfo .prj file (you may also need to copy it into the Discover .prj file for some Discover functions).  I have found that the resultant clause is generally accurate to 2 or 3m.  ( you can check the accuracy by snapping to one of your control points and looking at the on screen coordinates while viewing the map in your new projection) you can edit the 3rd (x direction adjustment) and 6th (y direction) numbers of the 6 numbers at the end of the clause  in the projection clause appropriately by the amount of error you found.  By doing this you can get accuracy to around 10cm.  You will also probably need to add bounds parameters to the end of the clause (eg 0,0,10000000,10000000) before pasting it into the .prj file.  If you don't have enough control points you can use your 2 control points to first generate a local grid layout in discover and then edit this file to the appropriate format and use it as your control file to generate the clause.  Hope this isn't too confusing.

 

 

Steve King

----- Original Message -----

From: Tom & Lisa Orr

To: mapi...@googlegroups.com

Sent: Tuesday, September 04, 2007 8:23 PM

Subject: [MI-L] Help with local grid projection