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<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff size=2>Hi
David,</FONT></SPAN></DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff size=2>I
would recommend keeping all the co-ordinates in geographics
(latitude/longitude), rather than UTM Zone 14. Keeping all the data in one zone
for an area that covers 3 to 5 UTM Zones does not make sense from a mapping
point of view or even from a data conversion point of view.</FONT></SPAN></DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff
size=2>Essentially your error is increasing exponentially the further away you
go from the Central Meridian in the UTM Projection. The more zones you cover,
the greater the error in the values. Not only that, as you transform the
coordinates back and forth from UTM 14 to geographic to UTM 12, 18, 15, etc.,
you will also be introducing errors again. Then if you reproject to a
different datum, say NAD83, you are introducing another error (even if you are
using a grid conversion, such as NADCON (larger error if you are using a
Molodensky conversion).</FONT></SPAN></DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff size=2>For
example, here in British Columbia, even using ArcSDE, coordinates
are stored in geographics (and we are covering 5 UTM Zones, but have
settled upon an Albers Projection - for mapping, and NAD83 for the datum). For
working in ArcSDE, take a look here: <A
href="http://srmwww.gov.bc.ca/gis/sdedata.html">http://srmwww.gov.bc.ca/gis/sdedata.html</A>.
Note that even the BC Gov't states that for working outside B.C., that
there "needs a different spatial reference, even if it is stored in
geographic or BC Albers projection. Data in this larger area must have a lower
resolution".</FONT></SPAN></DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff
size=2>Remember, coordinates that are stored in latitude and
longitude are usually stored as two real numbers in decimal degrees and we
know that these range from -180.0 W to +180.0 E in longitude, and -90.0 S to
+90.0 N in latitude, for a total of 360.0 degrees around the earth in each
direction. The earth's circumference is roughly 40,000 kilometres or 40,000,000
metres and therefore one can conclude there are about </FONT><PRE><FONT face=Arial color=#0000ff size=2>
40,000,000
----------
360</FONT></PRE><PRE><FONT face=Arial color=#0000ff size=2>
</FONT><FONT face=Arial color=#0000ff size=2>or about 111,000 metres per degree of latitude anywhere on the earth. The size of a degree of longitude varies - at the equator it's the same as a degree of latitude, but at the poles it is 0. </FONT></PRE>
<P><FONT face=Arial color=#0000ff size=2>9 significant digits is sufficient to
preserve metre precision (this would be number (9,6) in Oracle), or 11
significant digits to preserve centimetre precision (this would be number (11,8)
in Oracle).</FONT></P>
<P><SPAN class=800151616-07102007><FONT color=#0000ff><FONT face=Arial
size=2>Also note that by restricting the range (extent) of the area that can be
included in a dataset, it is possible to increase the precision of the
coordinates stored - therefore I would not store everything in UTM 14. I
would only keep the coordinates in UTM 14 if that was my only are of interest,
otherwise keep it geographic (latitude and longitude). UTM is LESS PRECISE after
all the conversions and especially since we are talking about 3 to 5
zones.</FONT></FONT></SPAN></P>
<P><SPAN class=800151616-07102007><FONT face=Arial color=#0000ff size=2>I would
also recommend that your GIS department move their data into NAD83/WGS84, not
NAD27. WGS84 is a precisely determined system, while NAD27 is not. NAD27 is
based on triangulations being joined together to form a model of the earth,
which is centered at Meades Ranch, Kansas, while WGS84 is ECEF (Earth Centered
Earth Fixed) and determined by Satellite Measurements. NADCON is an accurate
conversion for most mapping and GIS use, so you should be safe there. I would
also look at your local HARN network or any local geodetic work done by your
State Gov't Agency to see what they recommend.</FONT></SPAN></P>
<P><FONT face=Arial color=#0000ff size=2><SPAN class=800151616-07102007>Let me
know if I can expand further or answer more questions.</SPAN></FONT></P>
<P><FONT face=Arial color=#0000ff size=2><SPAN
class=800151616-07102007>Cheers,</SPAN></FONT></P>
<P><FONT face=Arial color=#0000ff size=2><SPAN
class=800151616-07102007>Dean</SPAN></FONT></P>
<P><FONT face=Arial color=#0000ff size=2><SPAN class=800151616-07102007>Dean C.
Mikkelsen, B.Sc., P.Eng.</SPAN></FONT></P>
<P><FONT face=Arial color=#0000ff size=2><SPAN class=800151616-07102007>Terra
ETL Ltd. - <A
href="http://www.terraetl.com">http://www.terraetl.com</A></SPAN></FONT></P>
<P><FONT face=Arial color=#0000ff size=2><SPAN
class=800151616-07102007></SPAN></FONT> </P></SPAN></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV></DIV>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left><FONT
face=Tahoma size=2>-----Original Message-----<BR><B>From:</B>
gdal-dev-bounces@lists.maptools.org
[mailto:gdal-dev-bounces@lists.maptools.org] <B>On Behalf Of </B>David M.
Baker<BR><B>Sent:</B> Sunday, October 07, 2007 8:40 AM<BR><B>To:</B>
gdal-dev@lists.maptools.org<BR><B>Subject:</B> [Gdal-dev] A Question of
Coordinate Storage<BR><BR></FONT></DIV>
<DIV class=Section1>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">All,<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">Though this question is not
specific to GDAL, OGR or FWTools, I believe some of the most knowledgeable
people of the subject are on this mailing list and hope you will allow me to
ask.<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">At my company, there is an ongoing
debate as to the best coordinate system to store cooperate spatial data
in. The debate is mainly between the Geoscience department and the GIS
department. The vast majority of the data the Geo’s use comes from
public sources that are in geographic lat/lon in NAD27. This data is
then reprojected for mapping and display in various coordinate systems, state
plane and UTM (zones 12 – 18, but mostly in 13, 14 and 15). The GIS guys
supply many layers to the Geo’s for display. They are currently
upgrading from ArcSDE 9.1 to 9.2, and in the process are converting all their
coordinates for storage in UTM zone 14. GIS made the comment that this
is a much more accurate way to store the data, that geographic coordinates are
less precise, though, their main reason for storing the data in a projected
system is to avoid having to do any coordinate conversion on the fly when
displaying the data on corporate web maps. The layers supplied to the
Geo’s will need to be reprojected if their current map projections are not UTM
Zone 14.<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">The question: is it wise to store
coordinates that cover such a wide area in a single projected coordinate
system, or would it be better to store them in some geographic coordinate
system, e.g., NAD83 or WGS84? And if this not the best way (UTM14),
why? What are the pitfalls? Is it really more accurate, or precise
to store data in a Cartesian coordinates and suffer the conversion to a
different projection as needed?<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">Thanks,<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial">David<o:p></o:p></SPAN></FONT></P></DIV></BLOCKQUOTE></BODY></HTML>