Coordinate Systems and Projections
Keith Fowlow
We have run into an issue with coordinate systems and projection and I am wondering if anyone can answer a few questions. My company work in Mexico, all over Mexico. We have data from each zone within Mexico. What we are going to encounter is trying to put data onto a map that may have been registered into different zones. Has anyone come across this in their work? A work around is just to put everything into Lat Long but it is nice to make maps for the field with UTM grids.
Another question is the Projections for Lat Long. There is a multitude of category members for Lat Long. My question is the difference between these members. Say for instance, is there a difference between Lat\Long NAD27 for Continental US and Lat\Long NAD27 for Canada? If so what is it and is there much of a change? The same goes for Lat\Long NAD27 Mexico and Lat\Long NAD27 Central America. There must be some sort of overlap there somewhere. Any help will br greatly appreciated.
While going through this I found a bug in the projection file for anything less than 7.8. It has to do with the numbers of the category member for Lat\Long. There are a couple of members that have the same number, i.e. Lat\Long NAD27 for Mexico and Lat\Long NAD27 for the Cental America. When you choose the Mexico projection, click okay and then go back into it, it change it to the Central America projection. If you look into the PRJ file they have the same number. There are two more in the PRJ file that have the same thing. Probably just typo. It is changed in 8.0
Cheers,
Keith
Phil
You would probably be better sticking with lat/long. Any co-ordinate
transformation is only an approximation which is valid for a specific
geographical area (hence multiple UTM projections for Mexico) and is
good enough for most purposes but when you try to edge match between
different co-ordinate systems, problems always occur. Mapinfo always
stores data internally in lat/long format and re-projects on the fly
to display a map in a different projection. This allows you to have a
map on the screen containing base data in several different
projections.
This is at the limit of my knowledge but your secondary question about
the different lat/long projections is a similar answer. Lat/long works
on the assumption that the earth is a true sphere that can be split
equally. In practice, it's not quite, it's slightly flattened at the
poles and bulges slightly at the equator (think of a fast spinning
tennis ball, the reason is much the same!). The local lat/long
projections are an attempt to even out these effects. WGS84 is treated
as the current reference ellipsoid (nope, not quite a sphere) and most
GPS equipment will be using this.
This link might be of interest:
http://en.wikipedia.org/wiki/World_Geodetic_System_1984
Regards,
Phil
www.philallen.net
Uffe Kousgaard
From: "Phil" <allen...@hotmail.com>
To: "MapInfo-L" <mapi...@googlegroups.com>
Sent: Monday, March 12, 2007 2:01 PM
Subject: [MI-L] Re: Coordinate Systems and Projections
> Mapinfo always
> stores data internally in lat/long format and re-projects on the fly
> to display a map in a different projection.
Not quite so. MapInfo stores coordinates in the actual coordinate system in
the TAB files, but uses integers for doing so in a clever way (not floating
point numbers). That's why you need to apply bounds for your TAB files.
A map based on the same coordinates as used in the underlying TAB file would
most likely not be re-projected at any time - it would be a waste of CPU
resources.
Re-projections between more than 1 coordinate system may very well involve
lat/long as a common format.
Regards
Uffe
GISgeek
I am not an expert, but I do work with data that is registered across
3 or more utm zones, when this happens and I want to display the data
on a large map, I use and ALBERS projection. I have created Custom
Projections for Alaska, British Columbia, and the Yukon. here is an
example...
"ALASKA_ALBERS", 9, 63, 7, -154, 50, 55, 65, 0, 0
The last six numbers are the the false origin,bounds and false easting
and northing. Create one for Mexico. If you do not have a custom
projection in your mapinfow.prj file, just add a space and a line "---
Custom ---" and add your new projection on a new line.
It might look like this
"ALBERS_MEXICO", 9, 72, 7, -120, 0, 10, 35, 0, 0
where
9=Projection=Albers equal area
72=Datum= NAD27 Mexico
7=Units=metres
-120,0=Origin in Lat/Lon
10, 35= Standard parallels=North and South of area
0,0=False Easting, false Northing
or you could check out page 454 of your Mapinfo v 8.0 User Guide!
Cheers,
Scott