<div dir="ltr">Hi Etienne,<div>I was looking for something like that but the <a href="http://www.gdal.org/frmt_gtiff.html">http://www.gdal.org/frmt_gtiff.html</a> page didn't list it explicitly.</div><div><br></div><div>
That said, looking at the gdalinfo now (a little late) I see that it's already defaulting to Float32. I also tested with the "-ot Float32" arguement, but there was no change. I'm counting 16 significant figures; I only want 5 or 6.</div>
<div><br></div><div>Cheers,</div><div>Jonathan<br><div class="gmail_extra"><div><br></div>
<br><br><div class="gmail_quote">On 10 March 2014 17:49, Etienne Tourigny <span dir="ltr"><<a href="mailto:etourigny.dev@gmail.com" target="_blank">etourigny.dev@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div dir="ltr">Have you tried using a 36bit (float) instead of 64bit (double) output? double is usually the default output type and has more significant digits (~16) than your source, so it's representation is "ok" in double format<div>
<br></div><div>see [1] for an explanation on floating-point precision</div><div><br></div><div>the following argument would do this "-ot Float32"</div><div><br></div><div>[1] <a href="http://en.wikipedia.org/wiki/Floating_point" target="_blank">http://en.wikipedia.org/wiki/Floating_point</a></div>
<span class=""><font color="#888888">
<div><br></div><div>Etienne</div></font></span></div><div class=""><div class="h5"><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Mar 10, 2014 at 2:25 PM, Jonathan Moules <span dir="ltr"><<a href="mailto:jonathanmoules@warwickshire.gov.uk" target="_blank">jonathanmoules@warwickshire.gov.uk</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr">Hi List,<div>Bringing back an old thread but with a slight change.</div>
<div><br></div><div>I'm now trying to get GDAL to mosaic and export from the ASCII files to a GeoTIFF.</div>
<div>But the GeoTIFF writer in GDAL is doing the exact same thing as the ASCII writer was - it's taking <b>50.597</b> (source value) and converting it into something like <b>50.596895342423</b>.</div>
<div><br></div><div>I've tried using both gdal_merge and going via a VRT (gdalbuildvrt - gdal_translate). The result is the same.</div><div><br></div><div>I also tried the <i>DECIMAL_PRECISION</i> trick, but gdal informs me:</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Warning 6: Driver GTiff does not support DECIMAL_PRECISION creation option</blockquote>
<div><div class="gmail_extra"><div><br></div>
How do I get GDAL to merge these into a GeoTIFF without altering the data?</div><div class="gmail_extra"><br></div><div class="gmail_extra">Thanks,</div><div class="gmail_extra">Jonathan<div><div><br><br><div class="gmail_quote">
On 24 January 2014 16:12, Jonathan Moules <span dir="ltr"><<a href="mailto:jonathanmoules@warwickshire.gov.uk" target="_blank">jonathanmoules@warwickshire.gov.uk</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr">Thanks Norman. I'd already seen that page but failed to read it!<div>
However, having tested it, it's not functioning as the manual says.</div><div><br></div><div>As best I can tell, DECIMAL_PRECISION=3 should give me 3 decimal places. But what it instead does it give me 3 significant figures. So 98.354 has become 98.3.</div>
<div>This becomes a problem because many of my heights are hovering around 100. So some will either have to be 1 character too long to ensure no data loss for the rest.</div><div><br></div><div>Is this a bug? The documentation on the page linked to is quite clear it should be decimal places.</div>
<div><br></div><div>Less of an issue, but it also didn't make any difference to:</div><div><div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
xllcorner 432000.000000000000<br>yllcorner 242000.000000000000<br>cellsize 2.000000000000</blockquote></div><div><br></div></div><div>GDAL 1.10.1, released 2013/08/26<br></div><div><br></div><div><br></div><div>
<div>Cheers,</div>
<div>Jonathan<div><div><br><div class="gmail_extra"><div><br></div>
<br><br><div class="gmail_quote">On 24 January 2014 15:11, Norman Vine <span dir="ltr"><<a href="mailto:nhv@cape.com" target="_blank">nhv@cape.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div style="word-wrap:break-word">try<br><blockquote type="cite"><div dir="ltr"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
gdal_translate -of AAIGrid -co DECIMAL_PRECISION=3 abc.vrt abc.asc</blockquote></div></blockquote><div>see <a href="http://www.gdal.org/frmt_various.html" target="_blank">http://www.gdal.org/frmt_various.html</a></div>HTH<div>
Norman</div><div><br><div><div><div><div>On Jan 24, 2014, at 10:01 AM, Jonathan Moules <<a href="mailto:jonathanmoules@warwickshire.gov.uk" target="_blank">jonathanmoules@warwickshire.gov.uk</a>> wrote:</div>
<br></div></div><blockquote type="cite"><div><div><div dir="ltr">Hi List,<div>I'm trying to mosaic some ASCII grid tiles into a single large ASCII file (which can then easily be handled).<br></div><div><br>
</div><div>I'm using this process:</div><div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
REM Create list of files<br>dir /b /s *.asc > asc_list.txt<br></blockquote><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
REM Turn into VRT<br>gdalbuildvrt -srcnodata "-9999" -vrtnodata "0" -a_srs "EPSG:27700" -input_file_list "asc_list.txt" abc.vrt<br></blockquote><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
REM actual mosaicing<br>gdal_translate -of AAIGrid abc.vrt abc.asc</blockquote></div><div><br></div><div>The input files are like:</div><div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
ncols 500<br>nrows 500<br>xllcorner 432000.000<br>yllcorner 243000.000<br>cellsize 2<br>nodata_value -9999.0<br>98.354 98.449 98.658 98.874 99.038 99.096</blockquote><div><br></div><div>But the outputs are like:</div>
<div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">fncols 500<br>nrows 1000<br>
xllcorner 432000.000000000000<br>
yllcorner 242000.000000000000<br>cellsize 2.000000000000<br>NODATA_value 0<br> 98.353996276855469 98.448997497558594 98.657997131347656 98.874000549316406 </blockquote></div><div>
<div><br></div><div>Suddenly everything has a dozen extra decimal places. Not only does this induce the False Precision problem (we don't have heights to the femtometer!), it also massively increases the filesize.</div>
</div><div><br></div><div>How can I get this to not happen?</div><div><br></div><div>Thanks,</div><div>Jonathan</div><div><br></div></div>
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