[R-sig-Geo] chull function behavior

[Carsten Neumann]

Dear List members,

sorry for the non specific geo question, but working with image spectra 
led me to the following problem calculating a convex upper hull curve 
using the function "chull {grDevices}".

Imaging there is a image spectra that you need to normalize using 
continuum removal (deviding spectra by upper hull), then you do not 
expect values greater than 1. Unfortunately the "chull()" functions did 
not identify all points of intersection on a hull curve. At the 
beginning there are also points beneath a hull, see example data (first 
3 points).

#############################################################################################################
wavelength<-seq(350,449,1)
dummy<-c(0.204054, 0.204828, 0.204161, 0.202444, 0.199883, 0.200960, 
0.203838, 0.205723,
  0.203404, 0.204835, 0.207257, 0.205682, 0.209670, 0.212622, 0.212032, 
0.211062,
  0.212727, 0.215129, 0.216259, 0.217435, 0.218270, 0.219039, 0.220333, 
0.222520,
  0.223502, 0.223225, 0.225050, 0.225485, 0.226832, 0.230962, 0.231891, 
0.231264,
  0.231170, 0.233540, 0.235799, 0.238165, 0.240494, 0.240793, 0.242218, 
0.244078,
  0.244795, 0.246343, 0.247142, 0.247406, 0.249348, 0.250772, 0.251937, 
0.253166,
  0.253301, 0.253941, 0.255231, 0.256503, 0.257881, 0.258632, 0.258916, 
0.260414,
  0.261584, 0.262598, 0.263891, 0.264702, 0.265681, 0.266892, 0.267861, 
0.268923,
  0.270050, 0.271112, 0.271767, 0.273130, 0.274604, 0.275065, 0.276157, 
0.277331,
  0.278321, 0.279585, 0.280577, 0.281469, 0.282497, 0.283157, 0.284381, 
0.285984,
  0.286866, 0.288173, 0.289387, 0.290084, 0.290770, 0.291659, 0.292730, 
0.293823,
  0.294841, 0.295763, 0.296607, 0.297420, 0.298529, 0.299691, 0.300580, 
0.301300,
  0.302290, 0.303378, 0.303885, 0.304686)

id<-sort(chull(wavelength, dummy))
plot(wavelength,dummy,type="l")
points(wavelength[id],dummy[id],type="p",col="red")
#############################################################################################################
Can anybody explain this behavior or give some advice on improving 
functionality?

Thanks in advance!

Best

Carsten


-- 

Carsten Neumann
Telegrafenberg, A 17 01.13
14473 Potsdam
-------------------------------------------------------
Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Department 1 - Section 1.4 - Remote Sensing
-------------------------------------------------------
Tel.: +49 (0) 331 288 1766
e-mail: carstenn at gfz-potsdam.de