Adding a EPSG:3785 datum to an older version of Mapinfo.

[phillit]

I have been using Mapinfo V8.5 for years and it has always worked fine until now that I'm trying to use aerial photography with a Popular Visualization CRS / Mercator (EPSG:3785).

I've dusted off the manual and updated the MAPINFOW.PRJ to add the datum using the below line

"--- CRS ---"

"Popular Visualization\p3785", 10, 157, 7, 0

I'm now getting an error saying "Invalid datum 157"

Reading through the manual further I saw the below reference for creating your own datum. Can someone suggest the correct datum figures for EPSG:3785 without buying a new version of Mapinfo?

9999, EllipsoidNumber, dX, dY, dZ, EX, EY, EZ, m, PrimeMeridian

[Tony Pilkington]

As far as I know, MI only “knows” about 2 spheres, The Google Sphere or the WGS84 Ball, and the authalic sphere used in all the spherical calculations, which is defined by having a an ID of 12 and a radius of 6370997.0m. Part of the 8 parameter definition of a datum is a scaling factor used to inflate or deflate a spheroid to make it fit.

 

In your case the scaling factor would be 6371204/6370997 = 1.00003249 giving a scaling  factor of 32.5 ppm. I have never found an example in MI literature of scaling factor in practical use.

 

So your prj line for the datum might look like: 9999 , 12 ,0 ,0 ,0 ,0 ,0 ,0, 32.5

 

STM1987 (DNAG)", 10, 9999 , 12 ,0 ,0 ,0 ,0 ,0 ,0, 32.5, 7,0 if it were Mercator on Google. The 7 can be replaced by “m”.

 

However, if precision is not too important, set the inflation factor to 0 and cross your fingers. It might be close enough.

 

Tony

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[phillit]

Hi Tony thankyou for your response. Can you please confirm the syntax on this command. I've added a new category called My Category and used your parameters but I'm getting an error of points cannot all be in one line.

"--- My Cateorgy ---"

"Popular Visualization\p3785", 9999 , 12 ,0 ,0 ,0 ,0 ,0 ,0, 32.5, "STM1987 (DNAG)", 10, 9999 , 12 ,0 ,0 ,0 ,0 ,0 ,0, 32.5, 7,0

[Bill Thoen]

Also note the 999 and 9999 at the start of the coordsys definition. If you're defining the 7 element version, you need to use 9999 to alert  MapInfo that a 7 (or an 8?) element custom definition is coming at it. Use  999 for 3 or 4 element custom CRS's.

This area of knowledge (custom coordsys' ) is covered in much more detail in the help system, and it's NOT in the printed manual.

[Tony Pilkington]

Update: The scaling factor calculated in my first reply below is for the Geological datum for North America, not the Google Sphere;

Using the Google Sphere we get:

 

6378137/6370997 = 1.001120703.... or 1121 ppm.

Since the latitude and longitude of a point will be the same using either sphere (think of an onion) and considering a distance of 1 minute of arc on the equator, we get a distance of:

radius *1/60 * radians/degree

 

for Google Sphere: 1855.324 metres.

for Authalic Sphere: 1853.248 metres.

 

or approximately 2m per minute of arc (or nautical mile which is defined at 1852 metres) or approx 0.1%

This equates to 8 pixels for 25cm resolution imagery.

 

Maybe PBMI would like to comment on the use of the Authalic sphere for Google imagery and whether their calculations of spherical distance and area take into consideration the sphere that is being used.

 

If any one could publish the UTM coordsys lines for Google Sphere (Datum 157), it would be very useful.

 

Footnote: Many publishers of rectified imagery use the Authalic sphere rather than the local ellipsoid for calculations for simplicity and speed.