[Qgis-user] Re: Odd Null value

[Agustin Lobo]

Peter,

I've set the NoData to the actual value
-340282000000000014192072600942972764160.00

as read thanks to the "Add values from display"
in the same Transparency tab,

but this does not seem to have any effect in
the histogram or in the min and max values
loaded in tab Symbology: the min is
still -3.40282e+38 . I understand
that if the user sets -3.40282e+38 as NoData,
this value cannot be considered the min. So
it seems that the program cannot use
-340282000000000014192072600942972764160.00
as NoData

I've ported the image (npp_geotiff) into R through manageR:

a <-  npp_geotiff at data$band1
 >summary(a)
       Min.    1st Qu.     Median       Mean    3rd Qu.       Max.
-3.403e+38 -3.403e+38 -3.403e+38 -2.391e+38  2.776e+10  1.156e+12

 >summary(a[a<0])
       Min.    1st Qu.     Median       Mean    3rd Qu.       Max.
-3.403e+38 -3.403e+38 -3.403e+38 -3.403e+38 -3.403e+38 -3.403e+38

So -3.403e+38 is the only negative value.

If I set:
 >a[a<0] <- NA
 >summary(a)
        Min.     1st Qu.      Median        Mean     3rd Qu.        Max.
6.29800e+08 4.56200e+10 1.21100e+11 2.32400e+11 3.07900e+11 1.15600e+12
        NA's
5.78832e+05

Then I've defined:
 >b <-  npp_geotiff
 >b at data$band1[b at data$band1  <0 ]<- 0

and passed b to QGIS through manageR

There I set 0 as No Data and get the same
behaviour, 0 is still considered as the min,
while (in my opinion) it should not,

Agus


Peter Ersts wrote:
> Agustin,
> 
> You will notice that that image does not have a valid NoDataValue by 
> default, which is why the field in the properties dialog is blank.
> 
> The transparency table and NoDataValue field can handle scientific 
> notation just fine. The problem is that -3.40282e+38 is NOT the "exact" 
> value of the pixel, so the no data and transparency checks fail.
> 
> Overly simplistic example
> ( 3.2e+01 == 3.28 ) is false.
> 
> The "value tool plugin", if that is what you are using to find the 
> values,  should let you choose between scientific notation and actual 
> value for output display. Maybe the documentation or metadata for the 
> project that produced that image will have a detailed description of 
> pixel values.
> 
> -pete
> 
> Agustin Lobo wrote:
>> I have a raster map
>> (http://sedac.ciesin.columbia.edu/es/npp_geotiff.zip)
>> with the odd value of
>> -3.40282e+38
>> as null value (this is probably the way some software is writing its 
>> null values)
>>
>> Is there any way I can set this value as NO DATA VALUE in QGIS?
>> I have the impression that the field in the Transparency tab does not 
>> accept the e+38
>> I've been able to set it in the transparency list thanks
>> to the tool that lets you select in the image, but then
>> cannot copy and paste.
>>
>> Thanks
>>
>> Agus
> 
> 

-- 
Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: Agustin.Lobo at ija.csic.es
http://www.ija.csic.es/gt/obster