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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p style='margin-left:.5in;text-indent:-.25in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Wingdings'><span style='mso-list:Ignore'>Ø<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>First question: how can one recode <b>gbr100x100d </b>so that all pixels with value 0 are NODATA?<o:p></o:p></p><p class=MsoListParagraph><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>Use ST_SetBandNoDataValue <a href="https://postgis.net/docs/manual-3.1/RT_ST_SetBandNoDataValue.html">https://postgis.net/docs/manual-3.1/RT_ST_SetBandNoDataValue.html</a><o:p></o:p></span></p><p class=MsoListParagraph><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>UPDATE gr100x100d SET ST_SetBandNoDataValue(rast,1,0);<o:p></o:p></span></p><p class=MsoListParagraph><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>General side note, using ST_Reclass for what you are doing below should be much faster than ST_MapAlgebra<o:p></o:p></span></p><p class=MsoListParagraph><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-size:11.0pt;font-family:Wingdings;color:#1F497D'><span style='mso-list:Ignore'>Ø<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'>Second question: I haven’t had a chance to look that closely. What is the purpose of the ST_SnapToGrid?<o:p></o:p></span></p><p class=MsoListParagraph><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'> postgis-users [mailto:postgis-users-bounces@lists.osgeo.org] <b>On Behalf Of </b>Simon SPDBA Greener<br><b>Sent:</b> Tuesday, March 16, 2021 8:15 PM<br><b>To:</b> postgis-users@lists.osgeo.org<br><b>Subject:</b> [postgis-users] PostGIS Raster: Two Questions<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><p>I have a 1 band raster, srid 4326, (0.001 x 0.001 roughly 100m x 100m) , called <b>gbr100x100d</b>, whose pixels are coded 0 and 1 where 1 is water deeper than 100m and 0 water shallower than 100m. <o:p></o:p></p><p>It was created as follows:<o:p></o:p></p><p><b><span style='font-family:"Courier New"'>create table gbr100x100d<br>as<br>SELECT row_number() over (order by 1) as rid,<br> ST_SetSrid(ST_MapAlgebra(a.rast, 1, NULL, '([rast.val]<-100.0)::integer'),4326) as rast<br> from gbr100x100c as a;</span></b><o:p></o:p></p><p>First question: how can one recode <b>gbr100x100d </b>so that all pixels with value 0 are NODATA?<o:p></o:p></p><p>Next. <o:p></o:p></p><p>Polygons of size 0.02x 0.01 need to be extracted from the 1 valued 0.001 pixels.<o:p></o:p></p><p>I think this should be done as follows.<o:p></o:p></p><p>First we resample to get a raster with cells of size 0.02 x 0.01:<o:p></o:p></p><p><b><span style='font-family:"Courier New"'>create table gbr2kmx1kmd1<br>as<br>SELECT row_number() over (order by 1) as rid,<br> ST_SetSrid(ST_Rescale(a.rast,0.02,-0.01,'Bilinear',0.125),4326) as rast<br> from gbr100x100d as a;</span></b><o:p></o:p></p><p>Then we extract the polygons of these 0.02 x 0.01 pixels.<o:p></o:p></p><p><b><span style='font-family:"Courier New"'>create table gbr2kmx1kmd1p<br>as <br>SELECT row_number() over (order by 1) as id, (gv).val, ST_SnapToGrid((gv).geom,0.001)::geography as geog<br> FROM (SELECT ST_PixelAsPolygons(a.rast,1,true) as gv <br> FROM gbr2kmx1kmd1 as a<br> ) gv<br> WHERE (gv).val = 1;</span></b><o:p></o:p></p><p>The result is close as the polygons are 0.02 x 0.01 but they are organised as a set of striped polygons where the columns don't touch. <o:p></o:p></p><p>See attached.<o:p></o:p></p><p>What was expected was a complete coverage of abutting polygons. <o:p></o:p></p><p class=MsoNormal>Can anyone spot what I am doing wrong? <o:p></o:p></p><p>regards<o:p></o:p></p><p>Simon<o:p></o:p></p></div><p class=MsoNormal><br>2021-03-17_10-27-06.png <o:p></o:p></p><div><p class=MsoNormal><img border=0 width=484 height=421 id="_x0000_i1025" src="cid:image001.png@01D71AA7.9D0E2710"><o:p></o:p></p></div></div></div></body></html>