<p dir="ltr">Hi,<br>
If the no data value is a problem, you can change it. Look at this post. If you are more comfortable with the GRASS and SAGA raster calculators, you can access them using the "processing plugin".</p>
<p dir="ltr">http://gis.stackexchange.com/questions/81640/how-to-set-all-pixels-with-value-0-to-nodata-in-dem-raster<br></p>
<p dir="ltr">Nicolas Cadieux M.Sc.<br>
Les Entreprises Archéotec inc. <br>
8548, rue Saint-Denis Montréal H2P 2H2<br>
Téléphone: 514.381.5112 Fax: 514.381.4995<br>
www.archeotec.ca</p>
<div class="gmail_quote">On Jun 1, 2015 6:09 PM, "robertlozar [via OSGeo.org]" <<a href="/user/SendEmail.jtp?type=node&node=5208898&i=0" target="_top" rel="nofollow" link="external">[hidden email]</a>> wrote:<br type='attribution'><blockquote style='border-left:2px solid #CCCCCC;padding:0 1em' class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br />I cannot figure out how to overcome the QGIS raster calculators rules.
<br /><br />I need to calculate the Leaf Area Index -LAI- from satellite image bands. The LAI is dependent on the Land Cover -LC -
<br /><br />Type. For each LC, I need to result in 3 mutually exclusive conditions:
<br />LAI > 0, then calculate the value
<br />LAI < 0, set the value to zero (or a vanishingly small positive number)
<br />Not current LandCover, set the value to nodata.
<br />Then each different LC will be exclusive of all others. So I will have to add many mutually exclusive areas together.
<br /><br />I have written the equations as above for each of the landcover types separately. All I have to do is add one to the
<br /><br />next to get a map of LAI for the three conditions and then for all the landcovers.
<br /><br />The problem is the QGIS Raster Calculator will not let me do the simple addition. It seems the reason is the nodata
<br /><br />from the landcover type is what QGIS Raster Calculator is what takes priority over the data in the addition and all I
<br /><br />get is a final map full of nodata. I cannot find a way of overcoming this "feature". I cannot believe such a simple
<br /><br />action is not possible in any GIS. Can someone point out where I am going wrong?
<br /><br />Thanks, Bob
<br /><br />Note-I have done this in GRASS and ArcGis and Iknow it can be done in SAGA and the the QGIS plugings. Since the .tif
<br /><br />format is the standard for QGIS, one should not have to put the images into ascii format as some have suggested or use
<br /><br />gdal_translate to get zeros and nodata separated (which does not work anyhow). I am asking specifically about the QGIS
<br /><br />Raster Calculator because I cannot believe it is not possible.
<br /><br />The actual equations I have been using look like this:
<br />case 1 - calculates LAI for LC code 41 above zero correctly:
<br />(("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) AND (("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) * ( 2.8 -102 *
<br /><br />"clip_RT_LC80190382014318LGN00_B04@1" + 28
<br />* "clip_RT_LC80190382014318LGN00_B05@1" - 41 *
<br />"clip_RT_LC80190382014318LGN00_B06@1" ) >= 0)) * ( 2.8 -102 * "clip_RT_LC80190382014318LGN00_B04@1" + 28 *
<br /><br />"clip_RT_LC80190382014318LGN00_B05@1" - 41 *
<br />"clip_RT_LC80190382014318LGN00_B06@1" )
<br /><br />case 2 - calculates LAI for code 41 below zero to a small value correctly:
<br />(("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) AND (("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) * ( 2.8 -102 *
<br /><br />"clip_RT_LC80190382014318LGN00_B04@1" + 28
<br />* "clip_RT_LC80190382014318LGN00_B05@1" - 41 *
<br />"clip_RT_LC80190382014318LGN00_B06@1" ) <= 0)) * .0000001
<br /><br />case 3 - set value for all codes NOT 41 to a vanishingly small number:
<br />(("nlcd_2011_benning_areawgs84z16.tif@1" != 41 ) * .0000002)
<br /><br />So the final equation for Code 41 should be (but is NOT) as follows because
<br /> 1. qgis does not allow you to add one layer to a second that is in the seconds nodata area [same for + AND and OR
<br /><br />between each] and
<br /> 2. ("your_raster" != -32768) * "your_raster" does not work because image data is UInt16 in which nodata value
<br /><br />cannot be addressed (this statement may work for other formats)
<br /><br />The equation for Code 41
<br />(( (("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) AND (("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) * ( 2.8 -102 *
<br /><br />"clip_RT_LC80190382014318LGN00_B04@1"
<br />+ 28 * "clip_RT_LC80190382014318LGN00_B05@1" - 41 *
<br />"clip_RT_LC80190382014318LGN00_B06@1" ) >= 0)) * ( 2.8 -102 * "clip_RT_LC80190382014318LGN00_B04@1" + 28 *
<br /><br />"clip_RT_LC80190382014318LGN00_B05@1" -
<br />41 *
<br />"clip_RT_LC80190382014318LGN00_B06@1" ) ) OR ((("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) AND
<br /><br />(("nlcd_2011_benning_areawgs84z16.tif@1" = 41 ) * ( 2.8 -102 * "clip_RT_LC80190382014318LGN00_B04@1"
<br />+ 28
<br />* "clip_RT_LC80190382014318LGN00_B05@1" - 41 *
<br />"clip_RT_LC80190382014318LGN00_B06@1" ) <= 0)) * .0000001 ) OR (("nlcd_2011_benning_areawgs84z16.tif@1" !=
<br /><br />41 ) * (.0000002 ) ) )
<br />
<br />
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