[GRASS5] 0 != no data

Tue Jun 19 05:48:12 EDT 2001

Hello Justin, hello all,

Shouldn't there be an introductory phrase reminding people what this NULL 
business is all about ? I would imagine some users out there might not fully 
understand what we're talking about.

Moritz

Quoting Justin Hickey <jhickey at hpcc.nectec.or.th>:

> Hello all
> 
> Sorry for the late reply, but I had other matters to take care of. As
> per Markus' suggestions, some text has been changed in the post I
> recommend we send to users. Please take a moment to review the post
> below.
> 
> Markus Neteler wrote:
> > thanks for your nice proposal. A few remarks:
> > 
> > - Perhaps (even for the new users), an example may be added to 
> >   demonstrate the 0 to be NULL thing
> 
> I'm not sure what you mean here. What kind of example are you thinking
> of, a classification example?
> 
> > - it should be pointed out, that GRASS 5 will keep the feature to 
> >   store NULLs (for those not carefully ready), perhaps in a bottom
> >   summary
> > - it should be pointed out, that GRASS 5 will keep the feature to
> >   convert individual values to NULL and back
> > - maybe in a bottom summary it should be explained, that, if
> >   eliminating the feature of 0 to be NULL, that both groups (1) and
> >   (2) will stay happy, only that a conversion step is required (or
> >   not?).
> 
> OK I've made some changes, see the example post below.
> 
> > BTW: Does anyone know a free poll server without adult images or 
> > annoying ads? I don't want that they catch all the users' email
> > adresses and sell them anywhere.
> 
> Sorry I can't help you here, but we could simply have them reply to the
> message or send their votes to me if we can't find a suitable poll
> site.
> 
> 
> ======================== Start of post ===============================
> 
> Subject: We need your opinion
> 
> Hello all users
> 
> Recently on the developers mailing list, there has been a discussion
> concerning the current implementation of NULL. When the NULL concept
> was
> first introduced and implemented there were basically 2 groups of
> users:
> 
> 1. Those who worked mostly with classes and had a large number of 
>    files with 0 set as NULL
> 
> 2. Those who started to use raster maps as representation of
>    continuous fields and 0 was a data value
> 
> The majority of the users were in group 1 and thus, the decision was
> made to keep the feature that 0 could be interpreted as NULL.
> 
> In the discussion on the developers list, some developers have
> indicated
> that since GRASS 5.0 has the concept of NULL (which is represented by
> non-zero values), it is confusing to also have the feature that 0 can
> be
> interpreted as NULL, especially to new users of GRASS. This feature has
> also caused some problems for some users who had greyscale aerial
> photographs stored as raster maps in GRASS 4.x, and then transferred
> them to GRASS 5.0. The problem was that the color black (value 0) was
> being interpreted as NULL causing them to believe that their data had
> been corrupted somehow.
> 
> In order to eliminate this confusion, a proposal was put forth on the
> list that 0 should never be considered as NULL in GRASS 5.0. Also, in
> order to facilitate those users who wish to use their GRASS 4.x files
> with the new GRASS 5.0, we would provide a script to assist them in
> converting the 0 values of their data to appropriate NULL values in
> GRASS 5.0. 
> 
> This script has not been written yet but the proposed usage would be
> that a user would run the script and provide a list of GRASS 4.x
> databases which contain raster files in which 0 should be interpreted
> as
> NULL. The script will automatically go through all locations and
> mapsets
> in each database and perform the conversion for all raster files. The
> only requirement for this script would be that the GRASS 4.x data would
> have to be maintained in proper GRASS databases. That is, all
> directories under a database will be locations, and all directories
> under a location will be mapsets. If a user has files that he/she does
> not want converted then either the files can be moved to a different
> database, or the user can provide a specific location, or mapset, or
> raster file to the script.
> 
> In summary, the proposal is to remove the feature of 0 being
> interpreted
> as NULL from GRASS 5.0. It is important to remember that GRASS 5.0 will
> store well defined NULL values (non-zero) and that GRASS 5.0 will
> provide the capability to convert any value to NULL or vice versa. Note
> that the elimination of this feature will satisfy both groups of users
> since both NULL values and 0 will be available to users. The only
> consequence of this removal is that a conversion process will be
> necessary for converting GRASS 4.x data into GRASS 5.0 data.
> 
> What we would like you to do is provide us with your opinion on whether
> we should keep the feature of 0 being interpreted as NULL. To make
> things a little easier, we would like you to reply to this post stating
> which of the following options (a or b) you wish to support.
> 
>     a) Keep the feature of 0 being optionally interpreted as NULL in
>        GRASS 5.0
>     b) Eliminate the feature from GRASS 5.0, but a script will be
>        provided to convert GRASS 4.x files to GRASS 5.0 format for
>        those who wish 0 to be NULL
> 
> Thank you for your time in this matter.
> 
> ========================= End of post ================================
> 
> Once again, any comments in the wording of the above post are welcome.
> 
> -- 
> Sincerely,
> 
> Jazzman (a.k.a. Justin Hickey)  e-mail: jhickey at hpcc.nectec.or.th
> High Performance Computing Center
> National Electronics and Computer Technology Center (NECTEC)
> Bangkok, Thailand
> ==================================================================
> People who think they know everything are very irritating to those
> of us who do.  ---Anonymous
> 
> Jazz and Trek Rule!!!
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