[GRASS-dev] r.reclass only for int maps

[Michael Barton]

On Oct 9, 2011, at 9:00 AM, <grass-dev-request at lists.osgeo.org> wrote:

> Date: Sun, 9 Oct 2011 03:36:53 +0100
> From: Glynn Clements <glynn at gclements.plus.com>
> Subject: Re: [GRASS-dev] r.reclass only for int maps
> To: Martin Landa <landa.martin at gmail.com>
> Cc: GRASS developers list <grass-dev at lists.osgeo.org>
> Message-ID: <20113.2245.545837.762166 at cerise.gclements.plus.com>
> Content-Type: text/plain; charset=us-ascii
> 
> 
> Martin Landa wrote:
> 
>> is there any reason why r.reclass does not support FP raster maps?
> 
> The immediate reason is that the underlying functionality in libgis
> doesn't support it. r.reclass merely creates the tables; it's libgis
> which uses them.
> 
> It would be a relatively simple matter to extend the existing
> functionality to allow reclassing an integer map to an FP map (i.e. 
> the reclass table could contain FP values instead of integers).
> 
> It would be somewhat more complex to support "reclassing" an FP map. I
> use quotes because the term implies that the data consists of discrete
> classes (categories). Ultimately, a reclass table is just that: a
> table, i.e. an array, and you can't use floats as array indices.
> 
> While there's no fundamental issue with defining a mapping whose
> domain is floating-point numbers (we already do this for colour
> tables), it is bound to be significantly slower than the integer case.
> 
> And probably less useful.
> 
> A mapping from integers to integers can be described by a look-up
> table, but a mapping from floats to either integers or floats can't be
> (at least, not realistically). So you have to decide upon the nature
> of the approximation: piecewise linear, cubic spline, rational spline,
> or something else entirely? Or do we merge r.mapcalc into libraster so
> that maps can be "reclassed" by arbitrary algebraic expressions?
> 
> -- 
> Glynn Clements <glynn at gclements.plus.com>

I originally raised this question with regard to discrete values maps whose values are floating point numbers, not maps in which floating point numbers represent a continuous field (e.g., as in an elevation map). 

An example might be a soil map with a limited number of discrete values for K-factor. 

Whether integers or floating point numbers are used, r.reclass only makes sense in the context of a map with a small number of discrete values. In that case questions about approximations do not really apply. It should just be a simple 1 to 1 mapping like we have for integers. 

Are the limitations you mention a function of the data type itself or more about what the cell values represent?

Michael 
____________________
C. Michael Barton
Director, Center for Social Dynamics & Complexity 
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University

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