Lat/Long Projection

[Harley]

Hello,
Is anyone familiar with Map Info's Lat/Long projection (datum 0)? I
am stumped as to how it works if it does not reference an ellipsoid or
datum.

All of our Map Info data archives are in this projection. I recently
had some trouble exporting data in this projection which prompted me
to start looking into it. I've been trying to read everything I can
online (and it seems a lot of other people have this same question) -
I saw it described as a chameleon projection where it assumes the
projection of whatever base layer you have open in your mapper. But
does that mean we are only supposed to use this version of Lat/Long in
conjunction with another projection?

Similarly, I noticed when I export using Universal Translator, that
the .prj file references a WGS 1984 Spheriod BUT MapInfo has a
separate projection Lat/Long (WGS 1984) which is different from just
the basic Lat/Long (datum 0) projection.

I am currently using Map Info Professional version 9.5.

Thank you for your help.

Best regards,
Harley

[Richard Greenwood]

On Thu, Sep 24, 2009 at 11:48 AM, Harley <heco...@gmail.com> wrote:
>
> Hello,
> Is anyone familiar with Map Info's Lat/Long projection (datum 0)?  I
> am stumped as to how it works if it does not reference an ellipsoid or
> datum.

First the semantics - Lat/Long isn't a 'projection'. A projection is
when you cast (project) a spherical object onto a plane. A projection
is generally an equation that converts between shperical coordinates
and planar coordinates. The sphere should be referenced to a datum,
which for the sake of discussion, can be thought of as the sphere's
origin in space.

When you define both a projection and a datum, you have a 'coordinate
system'. In other words, you have defined how to get from planar
coordinates to shperical coordinates, and you have defined where the
sphere is in space (the datum). I'm simplifying a bit, but I think
this simplified view is sufficient to answer the question.

If I want to convert a State Plane NAD27 coordinate to a UTM NAD83
coordinate three steps would be required:
1. convert the planar State Plane coordiante to Lat/Long.
2. move the Lat/Long from NAD27 datum to NAD83 datum.
3. convert the NAD83 LAT/Long to UTM.

MapInfo's basic 'Lat/Long' isn't associated with any datum, as you
have noticed. So if you convert a MapInfo Lat/Long to UTM NAD83 it
does not attempt to convert the datum (step 2 above). And it you
convert the same Lat/Long value to State Plane NAD27 it still doesn't
attempt to transform the datum. Which is why you referred to it as a
'chameleon projection'. Unfortunately, it isn't actually changing to
match the target datum, it's not changing at all. So I'd call it
'color blind'.

Your data is most surely associated with some datum. You need to find
out what it is and then use MapInfo's "save as" to save the generic
Lat/Long to Lat/Long xyz, where xyz is the correct datum. UT is making
a possibly incorrect assumption in casting a generic MapInfo Lat/Long
to a WGS84 Lat/Long. It has no way of knowing what datum your data is
on. Additionally, UT does not reproject data, nor do datum
transformations. UT converts between data formats, not coordinate
systems.

And a caveat: MapInfo's support for datums is pretty thin in this day
and age. It's not really adequate for sub-meter mapping. But that's a
topic for another thread.

Rich

--
Richard Greenwood
richard....@gmail.com
www.greenwoodmap.com

[Geomap]

I am as perplexed as you.

And in fact I have seen some very strange behaviour when tables using
the Lat/Long - Lat/Long projection are combined with tables that have
other projections in the same window.

For example, I have a polygon that is defined with UTM WGS84
coordinates. If I save a copy table using Lat/Long - Lat/Long
projection and open the copy in the same window, the shapes match
exactly. However, If I change the projection of the map window to a
projection that is not based on the WGS84 spheroid, the two polygons
move apart. The difference in most cases is only a few metres, but I
am working in Laos where one of the most common projection is UTM with
Indian 1960 datum. With the map window in this projection, the
polygons will move apart by over 400m!

Regards
Nick

[Geomap]

I have just uploaded a file to demonstrate the behavior I described.
It contains the necessary files and a workspace to open windows in the
different projections.

Since Indian 1960 is not in the standard .prj file for Mapinfo, I have
also included a text file with the necessary lines to add the datum.
Just open your mapinfow.prj file in a text editor and paste in the
lines of text. Save and close the .prj file then restart Mapinfo. The
file should be at C:\Program Files\MapInfo\Professional\MAPINFOW.prj

Regards
Nick

On Sep 25, 12:48 am, Harley <hecoo...@gmail.com> wrote:

[Lars I. Nielsen (GisPro)]

Just wanting to rectify an error in Rich's post:

> Additionally, UT does not reproject data, nor do datum transformations.

UT is "FME Light", and both UT and FME reprojects between both projections and datums. In some cases, it's actually better at it than Pro.

Best regards / Med venlig hilsen
Lars I. Nielsen
GIS & DB Integrator
GisPro

Richard Greenwood skrev: