Projection Question (Again)

[Tim]

I'm really TRYING to understand how to use/build projection data...
BUT I'm still a little lost!

Some time ago Bill Wemple was kind enough to help me with a specific
problem that I had in Florida. He also provided a very concise
explaination of where he came up with the codes in the projection
"string."

I've gone through that explaination... compared it with Chapter 14 of
the MI 8.5 user's guide... and WAS feeling confortable with the data.

NOW I'm trying to work with some SHP files from British Columbia....
and am LOST AGAIN.

The projection files says...

GEOGCS["GCS_North_American_1983_CSRS98",DATUM["D_North_American_1983_CSRS98",SPHEROID["GRS_1980",
6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",
0.0174532925199432955]]

The METADATA tells me

Spatial Reference Information:
Horizontal Coordinate System Definition:
Geographic:
Latitude Resolution: 0.0000001
Longitude Resolution: 0.0000001
Geographic Coordinate Units: Decimal degrees
Geodetic Model:
Horizontal Datum Name: NAD83CSRS (North American Datum 1983 in
Canadian Spatial Reference System)
Ellipsoid Name: GRS80 (Global Reference System 1980)
Semi-major Axis: 6378137
Denominator of Flattening Ratio: 298.257222101

Would someone be willing to give me another explanation of how to
break down this information into a projection I can use with MapInfo.

Thanks!!!

Tim Ericson
Network Engineer
Qwest Communications

[Knightscape]

The PRJ goes something like this:

GEOGCS["GCS_North_American_1983_CSRS98", ----------------The
GEOGCS tells you that it's a Geodetic system (lat/long rather than
projected which is PROJCS)
DATUM["D_North_American_1983_CSRS98", ----------------Tells
you the name of the Datum it's based on. In this case NAD83-CSRS98
(used in Canada, considered equal to NAD83 at the +/-1m level)
SPHEROID["GRS_1980",6378137,298.257222101]], ---------------Tells
you the parameters of the Ellipsoid used in the datum.
PRIMEM["Greenwich",0],
----------------Tells you the prime meridian used for the datum.
UNIT["Degree", 0.0174532925199432955]] ---------------
Tells you the units for the system, and the conversion to meters.

There are only two variables in the definition for a Lat/Long system
in MapInfo. For lat/long the units are usually going to be degrees
(1), and the datum corresponds to NAD83 (74). All of the above would
line up with this [existing] line of the MAPINFOW.PRJ file:

"Longitude / Latitude (NAD 83)\p4269", 1, 74

Hope that helps a little.

Sam Knight

Applications Specialist
Blue Marble Geographics

geo...@bluemarblegeo.com
Ph: 800-616-2725 ext. 24
Ph: 207-582-6747 ext. 24
Fax: 207-582-7001
http://www.bluemarblegeo.com
and
http://www.beyondgeo.com