[GRASS-dev] CELL/FCELL/DCELL [was: Re: r.mapcal rand() strangeness]

[Helena Mitasova]

I very much agree with Glynn on this one, I have already written to  
Maciek
something along this line.

If you are going to explain in GRASS man pages how computers handle
numbers should we then also explain (e.g. as part of r.slope.aspect),
how elevation is measured, what is the accuracy for different  
technologies,
what is the impact of resolution on slope estimate etc. - there would  
be no end,
because you would need to cover not only GIS, computer science,  
remote sensing,
physics .....

BTW the info about CELL, FCELL and DCELL is in the man pages.

Helena

P.S. And regarding the requirement that more 16 digits are supported
that got all of this started - where do you need it? I am sure there are
applications where you do, especially in numerical simulations but as
an example (if I am not wrong) when you measure distance between
moon and earth with micron accuracy you would need that many digits.


On Feb 27, 2008, at 8:09 AM, Glynn Clements wrote:

>
> Hamish wrote:
>
>>> Below follow details about CELL and DCELL datatypes in GRASS. It
>>> would be good to have them summarrised in GRASS raster intro IMHO;
>>> + FCELL specific notes.
>> ...
>>>>>> A double has a precision of ~16 decimal digits, which matches
>>>>>> what you see above.
>>
>>
>> this stuff seems (to me) a bit too technical for the short "hello
>> raster" intro pages. is there another place we could put it?
>
> 	http://en.wikipedia.org/wiki/Floating-point
> 	http://en.wikipedia.org/wiki/IEEE_754
>
>> libgis Doxygen comments -> the (under advertised) programmer's  
>> manual?
>> maybe that is too far away from the user..?
>
> It doesn't belong in the programmer's manual any more than tutorials
> on the C language or the Unix API do.
>
> FWIW, the help file for Windows' calc.exe doesn't explain it either.
>
>> to take the idea further, when do you end describing how binary
>> computers deal with storing non-binary numbers? sure explain what  
>> GRASS
>> has used and the limits that imposes, maybe on a wiki page with links
>> to wikipedia articles on the general gotchas of modern C, x86,
>> endianness, etc. which may be natural to a computer scientist but
>> foriegn to an ecologist expecting a number to be number?
>
> Where do you stop? Do we include tutorials on using bash (or cmd.exe
> for Windows users), Unix filesystem permissions, grep/sed/awk (those
> are useful for processing input and output to/from various GRASS
> commands), statistics, ...?
>
> -- 
> Glynn Clements <glynn at gclements.plus.com>
> _______________________________________________
> grass-dev mailing list
> grass-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-dev