[GRASS5] 0 != no data

[Helena Mitasova]

Roger's introduction really helps to clarify what the message is all about.
I would just slightly modify the following sentence, to make it clear that
only
those GRASS4.* files/databases which use 0 as "no data" need to be converted.

------
Also, in order to facilitate those users who wish to use their GRASS 4.x
files which
use 0 as "no data" 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.
------

Helena

Roger Bivand wrote:

> Justin and friends,
>
> Would this modified version answer Moritz' question? I agree with him that
> we might risk getting a limited response because a regular 4.* user
> mightn't grasp the issue. I haven't got a 4.3 installed, but as far as I
> remember, there aren't any NULL values in the 4,* series are there? Can
> someone please check? I've also added a third, compromise, choice - remove
> 0 as NULL from 5.1, but retain it in 5.0: the motivation is to lower the
> threshold for 4.* users migrating to 5.*. I shortened the draft by taking
> out the script details - they are valuable, but could be included as a
> link to a note on the website.
>
> Roger
>
> ------------------------------------------------------------
> Subject: Representation of "no data": we need your opinion
>
> Hello all users
>
> Could we please ask you to review the issue of the representation of
> "no data" described below and give us your views on the alternatives we
> propose? This affects users of GRASS 4.* considering moving to GRASS 5.*,
> and users of GRASS 5.0 beta who depend on using legacy data. This is your
> chance to influence a major decision which may impact your use of GRASS!
>
> Recently on the developers' mailing list, there has been a discussion
> concerning the current representation of "no data" in raster map
> layers. When working with categories/classes, it was convenient to use
> integer zero (0) to express "no data", because the classes each had
> non-zero numbers - like land-use classes. This is the position in GRASS
> up to 4.*. GRASS 5.0 beta integrated experimental work done over many
> years by people needing both floating-point raster values, and needing
> to use numeric zero as a number.
>
> This meant that representing "no data" raised a conflict of interest:
> people with legacy data didn't want their "no data" (0) to "come back to
> life", while the numerical people really needed zero, as in zero slope,
> to have a meaning other than "no data". The term introduced to cover
> "no data" is "NULL", which can include both "no data available", and
> other situations where no numeric representation exists, like log of a
> negative number, or dividing by zero.
>
> There were then basically 2 groups of users in the mid 1990's:
>
> 1. Those who worked mostly with classes and had a large number of
>    files with 0 set as "no data"
>
> 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 "no data".
>
> In the discussion on the developers' list, some developers have indicated
> that, since GRASS 5.0 implements the concept of NULL (which is represented
> by machine dependent constants), it is confusing to also have the feature
> that 0 can be interpreted as NULL, especially to new users of GRASS. NULL
> is implemented as a separate mask of bits representing "data"/"no data",
> in principle for each raster map layer, but not all modules generate them
> automatically, meaning that users not knowing the history get mixed up.
>
> In order to eliminate this confusion, a proposal was made that 0
> should never be considered as "no data" 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.
>
> 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 "no data". To make
> things a little easier, we would like you to reply to this post stating
> which of the following options you wish to support:
>
>     a) Full legacy interoperability: keep the feature of 0 being
>        optionally interpreted as "no data"
>
>     b) Transition from GRASS 5.0 onwards: keep the feature of 0 being
>        optionally interpreted as "no data" in GRASS 5.0 only, eliminating
>        it in subsequent releases
>
>     c) Transition now: eliminate the feature from GRASS 5.0, but provide
>        a script 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.
>
> ------------------------------------------------------------
> --
> Roger Bivand
> Economic Geography Section, Department of Economics, Norwegian School of
> Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
> Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
> e-mail: Roger.Bivand at nhh.no
> and: Department of Geography and Regional Development, University of
> Gdansk, al. Mar. J. Pilsudskiego 46, PL-81 378 Gdynia, Poland.
>
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