[R-sig-Geo] spacetime : adding a fourth (spectral) dimension

[Edzer Pebesma]

On 05/04/2011 12:30 AM, scizmeli wrote:
> I am reflecting on a data structure that will allow us to perform all kinds
> of spectral operations we normally do in remote sensing. Due to lack of an
> appropriate data structure supporting spatially based spectral data, right
> now we have to keep our data in non-georeferenced N-dimensional arrays. As
> for extending spacetime, for now, I can think of an ability to:
> 
> * store in the same spacetime object both the spectral and the non-spectral
> products (a spectral radiance image cube+computed image layers);

You can do that already.

> * easily subset ranges of wavelengths (hopefully both by wavelength and band
> indexes) and be able to easily perform all kinds of space-time operations on
> the subsets;

you can by number and name, not by wavelength. An issue to consider is
that wavelengths for bands are typically intervals, so selecting a range
of values, say 500-600 nm, is ambiguous about whether to include a band
with range 490-510 yes or no. It would not be that hard to
implement; xts lets you select by time range, e.g. "2006" will select
all observations in that year.

> * select pixels and extract full (or partial) spectra

If you mean pixels in space-time, you can, sort of, do that; some
manipulation may be needed for the result to be understood as a
spectrum, and plotted nicely.

> * easily plot different spectral images in lattice panels (with stplot?) for
> a given time

This would not be too hard.

> * some channels are usually too noisy so we would need ability to flag some
> unwanted channels at some pixels
> * store spectral data for point and trajectory measurements

That is foreseen in the next few months.

> * incrementally import high volumes of data without having to load at once
> all the data in memory. Size of spectral image times series can be very
> large. On-disk storage can be sought with the use of netcdf technology for
> extremely large databases.

This was identified as one of the priorities on the workshop we
organized last March (www.opengeostatistics.org). I'm not sure how
fast such a feature will arrive.

> 
> and ability to easily implements methods like :
> 
> * band-ratio (or all kinds of other arithmetic) operations to compute new
> image layers.
> * computation of things like spectral derivatives, spectral unmixing as well
> as classification algorithms that use spatial/spectral information together
> etc.
> * overlay ground truth data (ship trajectory/ground truth points/polygons)
> onto image time series to match image pixels with ground truth data based on
> spatial coordinates and time information (ship trajectories will coincide
> with satellite images only at some given days)
> * easily plot different spectra (graph of measurement wrt wavelenth) for a
> given pixel in panels (or in the same graph) for different times (with
> stplot?)
> * spectrally interpolate images with a high spectral resolution  to estimate
> non-existing channels
> 
> There will certainly be dimensions other than "spectral" that people from
> other disciplines will want to add in a spacetime object. So, to my mind, it
> all boils down to the ability of adding custom dimensions into a spacetime
> object (with numeric/string attributes addressable as in 
> st[pixel,time,wavelength,otherdimension]
> 
> If the upcoming versions of the spacetime package can be designed in such a
> way, we could start implementing methods  on spacetime objects for our tasks
> in hand and the package would very fast become popular.
> 
> Are any these ideas feasible?

I think so. What is lacking is a large amount of enthousiasm,
use cases, and programming efforts!

> Servet
> 
> PS: I am definitely ready to help if you think this is a good direction to
> go. I never created new object/methods myself but I am committed to learn
> pretty fast if some initial directions are given.

O.K.; then I'd recommmend the book "S programming", or something
similar, to get up to speed, or one of John Chambers' books on
R programming.

Best wihses,

> 
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de