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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Dean,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Thanks for the link you provided.
There is a lot on info on this subject germane to my problem. Please see
a reply I made to Frank earlier. We are not proposing storing our
corporate data in NAD27 even though most of us geoscientists do in our local
database (because most of our vendor data come in NAD27). I guess I need
to do some testing similar to those referred to here: <a
href="http://srmwww.gov.bc.ca/gis/bceprojection.html">http://srmwww.gov.bc.ca/gis/bceprojection.html</a>.
The GIS guys claim greater accuracy, though I think they mean precision, but
the fact is our data is not that accurate. If we are within a meter we
are doing well, though if I new for sure we were within 5 meters I would be
happy. We have old data that is measured off a Jeffersonian gird system
and converted to NAD27 lat/lons, we have newer, survey, data that referenced to
state plain and then converted to NAD27 lat/lon, but more recently we get data
that the positions are acquired by GPS and reported in WGS84. So we start
with a real provenance problem, but we live with that. What concerns me
is potential for compounding or needlessly adding error for the sake of speed
in our map display.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>David<o:p></o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> Dean C.
Mikkelsen [mailto:dcmikkelsen@shaw.ca] <br>
<b><span style='font-weight:bold'>Sent:</span></b> Sunday, October 07, 2007
11:55 AM<br>
<b><span style='font-weight:bold'>To:</span></b> 'David M. Baker';
gdal-dev@lists.maptools.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> RE: [Gdal-dev] A Question
of Coordinate Storage</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Hi David,</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>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.</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>Essentially your error is increasing
exponentially the further away you go from the <st1:place w:st="on">Central
Meridian</st1:place> 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).</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>For example, here in <st1:State w:st="on"><st1:place
w:st="on">British Columbia</st1:place></st1:State>, 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".</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=2 color=blue face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:blue'>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 </span></font><o:p></o:p></p>
<pre><font size=2 color=blue face=Arial><span style='font-size:10.0pt;
font-family:Arial;color:blue'> <o:p></o:p></span></font></pre><pre><font
size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:blue'> 40,000,000 <o:p></o:p></span></font></pre><pre><font size=2
color=blue face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:blue'> ---------- <o:p></o:p></span></font></pre><pre><font
size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:blue'> 360</span></font><o:p></o:p></pre><pre><font
size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:Arial;
color:blue'><o:p> </o:p></span></font></pre><pre><font size=2 color=blue
face=Arial><span style='font-size:10.0pt;font-family:Arial;color:blue'>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. </span></font><o:p></o:p></pre>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>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).</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>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.</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>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, <st1:State w:st="on"><st1:place
w:st="on">Kansas</st1:place></st1:State>, 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.</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>Let me know if I can expand further or answer more questions.</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>Cheers,</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>Dean</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>Dean C. Mikkelsen, B.Sc., P.Eng.</span></font><o:p></o:p></p>
<p><font size=2 color=blue face=Arial><span style='font-size:10.0pt;font-family:
Arial;color:blue'>Terra ETL Ltd. - <a href="http://www.terraetl.com">http://www.terraetl.com</a></span></font><o:p></o:p></p>
<p><font size=3 face="Times New Roman"><span style='font-size:12.0pt'> <o:p></o:p></span></font></p>
</div>
<blockquote style='margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt'>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=2 face=Tahoma><span
style='font-size:10.0pt;font-family:Tahoma'>-----Original Message-----<br>
<b><span style='font-weight:bold'>From:</span></b>
gdal-dev-bounces@lists.maptools.org
[mailto:gdal-dev-bounces@lists.maptools.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>David M. Baker<br>
<b><span style='font-weight:bold'>Sent:</span></b> Sunday, October 07, 2007
8:40 AM<br>
<b><span style='font-weight:bold'>To:</span></b> gdal-dev@lists.maptools.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> [Gdal-dev] A Question of
Coordinate Storage</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>All,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
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 size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
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 size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
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 size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Thanks,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>David<o:p></o:p></span></font></p>
</blockquote>
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