Andrew<br><br>NAD27 is treated by a datum shift grid (or several grids as you can see in the output with -v)<br><br># +proj=lonlat +datum=NAD27 +ellps=clrk66<br>
# <a href="mailto:+nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat" target="_blank">+nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat</a><br><br>What the grids do is to estimate (delta_lon,delta_lat) to transform from NAD27 to WGS84. Since your input point (-62.16775,16.74132) is outside of the grid range, the shift is zero. So there is no change in the input coordinates, and accidentally your result is correct.<br>
<br>Regards<br><br>Jose Gonçalves<br><br><br><br><div class="gmail_quote">2012/2/3 Andrew Simpson <span dir="ltr"><<a href="mailto:drew.simpson.home@gmail.com">drew.simpson.home@gmail.com</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>Thank you for the responses Frank and Jose.</div><div> </div><div>So when I do the UTM to lonlat with no datum, the conversion the results are in WGS84.</div><div> </div><div>When I go from lonlat to epsg:2004, cs2cs assumes there is no datum conversion needed, so I get the wrong results.</div>
<div> </div><div>But if I specify a source datum of NAD27 or WGS84 on the latlon, the towgs84 in the target epsg:2004 kicks in?</div><div> </div><div>Not sure why I can put either datum in and get the same result?</div><div>
</div><div>Thanks again,</div><div>Drew<br><br></div><div class="HOEnZb"><div class="h5"><div class="gmail_quote">On Thu, Feb 2, 2012 at 6:22 PM, Jose Gonçalves <span dir="ltr"><<a href="mailto:jagoncal@gmail.com" target="_blank">jagoncal@gmail.com</a>></span> wrote:<br>
<blockquote style="margin:0px 0px 0px 0.8ex;padding-left:1ex;border-left-color:rgb(204,204,204);border-left-width:1px;border-left-style:solid" class="gmail_quote">Andrew<br><br>Since you don't specify an ellipsoid in the source system, WGS84 is assumed:<div>
<br><br>cs2cs -v +proj=lonlat +to +init=epsg:2004<br><br></div>This is not enough because you must specify some datum control, such as +towgs84=0,0,0 (notice that +datum=WGS84 is equivalent to +ellps=WGS84 +towgs84=0,0,0). You get the correct result with:<br>
<br>cs2cs +proj=lonlat +towgs84=0,0,0 +to +init=epsg:2004<br>-62.16775 16.74132<br>381799.69 1850374.10 37.86<br><br><br>Regards<br><br>Jose Gonçalves<br><br><br><br><br><div class="gmail_quote">2012/2/2 Andrew Simpson <span dir="ltr"><<a href="mailto:drew.simpson.home@gmail.com" target="_blank">drew.simpson.home@gmail.com</a>></span><br>
<blockquote style="margin:0px 0px 0px 0.8ex;padding-left:1ex;border-left-color:rgb(204,204,204);border-left-width:1px;border-left-style:solid" class="gmail_quote"><div><div><div>Greetings,</div><div> </div><div>
I am having trouble understanding how lonlat is handled in cs2cs and proj</div><div>
</div><div>If I cs2cs straight from "WGS 84 / UTM zone 20N" to "Montserrat 1958 / BWI" I get one result.</div>
<div> </div><div>If I cs2cs to lonlat in between I get a different result.</div><div> </div><div>If I provide a datum on the conversion from lonlat to "Montserrat 1958 / BWI" such as WGS84, I get the original result, even if the datum is not applicable as in NAD27.</div>
<div> </div><div>Is the datum enabling a portion of code that wouldn't otherwise be used, and then just being discarded?</div><div> </div><div>What EPSG code would lonlat default to, or does it default to the +to coordinate system?</div>
<div> </div><div>Thanks for any thoughts</div><div> </div><div>Rel. 4.7.1, 23 September 2009 on Linux 2.6.26-2-686 #1 SMP Thu Sep 16 19:35:51 UTC 2010 i686 GNU/Linux</div><div> </div><div> </div><div>====================================</div>
<div>-- All the way from UTM to Monty</div><div>cs2cs -v +init=epsg:32620 +to +init=epsg:2004 <<EOF<br>588710.63 1851124.52<br>EOF</div><div># ---- From Coordinate System ----<br>#Universal Transverse Mercator (UTM)<br>
# Cyl, Sph<br># zone= south<br># +init=epsg:32620 +proj=utm +zone=20 +ellps=WGS84 +datum=WGS84 +units=m<br># +no_defs +towgs84=0,0,0<br>#--- following specified but NOT used<br># +ellps=WGS84<br># ---- To Coordinate System ----<br>
#Transverse Mercator<br># Cyl, Sph&Ell<br># +init=epsg:2004 +proj=tmerc +lat_0=0 +lon_0=-62 +k=0.9995000000000001<br># +x_0=400000 +y_0=0 +ellps=clrk80 +towgs84=174,359,365,0,0,0,0 +units=m<br># +no_defs<br>381799.69 1850374.10 37.86</div>
<div>=====================================<br> <br>-- Inverse Proj from UTM</div><div>invproj -v -f "%.5f" +init=epsg:32620 <<EOF<br>588710.63 1851124.52<br>EOF</div><div>#Universal Transverse Mercator (UTM)<br>
# Cyl, Sph<br># zone= south<br># +init=epsg:32620 +proj=utm +zone=20 +ellps=WGS84 +datum=WGS84 +units=m<br># +no_defs +towgs84=0,0,0<br>#--- following specified but NOT used<br># +ellps=WGS84<br>-62.16775 16.74132</div>
<div> </div><div>======================================</div><div>-- lonlat to Monty</div><div>cs2cs -v +proj=lonlat +to +init=epsg:2004 <<EOF<br>-62.16775 16.74132 <br>EOF</div><div># ---- From Coordinate System ----<br>
#Lat/long (Geodetic)<br>#<br># +proj=lonlat +ellps=WGS84<br># ---- To Coordinate System ----<br>#Transverse Mercator<br># Cyl, Sph&Ell<br># +init=epsg:2004 +proj=tmerc +lat_0=0 +lon_0=-62 +k=0.9995000000000001<br>
# +x_0=400000 +y_0=0 +ellps=clrk80 +towgs84=174,359,365,0,0,0,0 +units=m<br># +no_defs<br>382121.20 1850599.03 0.00</div><div> </div><div>======================================</div><div>-- lonlat with a datum to Monty</div>
<div>cs2cs -v +proj=lonlat +datum=NAD27 +to +init=epsg:2004 <<EOF<br>-62.16775 16.74132 <br>EOF</div><div># ---- From Coordinate System ----<br>#Lat/long (Geodetic)<br>#<br># +proj=lonlat +datum=NAD27 +ellps=clrk66<br>
# <a href="mailto:+nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat" target="_blank">+nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat</a><br># ---- To Coordinate System ----<br>#Transverse Mercator<br># Cyl, Sph&Ell<br>
# +init=epsg:2004 +proj=tmerc +lat_0=0 +lon_0=-62 +k=0.9995000000000001<br>
# +x_0=400000 +y_0=0 +ellps=clrk80 +towgs84=174,359,365,0,0,0,0 +units=m<br># +no_defs<br>381799.69 1850374.10 37.86</div>
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