"Willy from Denmark" said:
> I have found that you should never, ever use the Lat/Long without datum.
> Apparantly MI being an US application then thinks that it implies, that
the
> NAD 27 was used in calculation of the coordinates.
Incorrect, see below.
Dan Page then asked:
> Is this correct that if a table is in Lat/Long projection with no datum
> associated in the projection list , MI will imply that it is in NAD27 and
> recalculate the coordinates from Nad27 to the new selected projection?
Don't worry; that isn't really the case. I tried several experiments,
starting
with a table that I knew was in a projection based upon NAD27.
I saved that table as lat/long, no datum, and then opened it in the
NAD27-based
map window I had. Sure enough everything lined up.
I saved the lat/long copy as a third copy, in lat/long with WGS84.
Sure enough, when I opened it in the NAD27-based map window, the WGS84 map
didn't line up with the other two.
"Gee, they're right", I said to myself. For a moment.
Then I tried another experiment: I created a map window from the
lat/long-no-datum
table and the lat-long-WGS84 table. The two layers lined up!
Then, when I changed the projection of my original map from a NAD27-based
one
to lat/long-WGS84, the two lat-long tables lined up, and it was the NAD27
table
that didn't line up.
Then, I changed the map window's projection to a lat/long based upon a
completely
different datum (Wake/Eniwetok, guaranteed to be far different from NAD27).
Nothing lined up!
Finally, I exported the lat-long-no-datum table and the lat-long-WGS84 table
to MIF/MID.
The decimal coordinates in the two MIF files were identical.
So remember:
1. In addition to each table having a projection, each map window has its
own projection. When you create a map, its projection is the projection of
the table
that makes its first layer.
2. Lat/long based upon no datum means "I don't know the datum, I'll go
along with
whatever else is there."
Because of this:
3. A lat/long-no-datum table is drawn in *whatever datum* the map window
uses.
4. When you save a copy of a lat-long-no-datum table into a
lat-long-with-a-datum
table, it simply substitutes the new datum; it does not change any
coordinates.
HTH
Spencer
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