I.POINTS

[charnotw]

> 
> Good Morning,
> 
> I'm interested to know if anyone has used i.points in Grass to register 
> SPOT , LANDSAT(MSS) or LAndSAT TM data. I have a couple of questions. 


we use i.points a lot on TM and have found it very satisfactory


> 
> 1) In the documentation on i.points it talks about having a map layer and the 
>    image you want to register. Then by looking at the "map" you can plot 
>    points on the image , minimum 3 , and register it that way. My question 
>    is does grass then take these points and extrapolate values over the 
>    whole image ? 
> 
>    And if so surely the accuracy of this will depend on the user knowing 
>    where on the image the referenced points are by looking at "map" layer. 
>    From the resolution of the images I have pulled into GRASS I can't see 
>    that this is going to be an accurate way of doing things , even with the 
>    "zoom" feature - Any other ways of doing it ?

generally you work on a group of images (specified using i.group) which
have been imported into grass with simple x,y coordinates.  i.points
provides the data that allows i.rectify to transform this coordinate 
system to a coordinate system in another map set. ie the transformation 
function is calculated from the points you specify in i.points. 

i.points operates in two ways: either you pick points off your image
and corresponding points from a different image in your target mapset
(just using the mouse alone). Or you can pick points off the source image
with the mouse and input the real world coords using the keyboard
(this is my preferred method)

you can't really expect on accuracy greater than within one or
two pixels because you can pickout features any smaller.
 
> 2) Once the image is registered in needs to be rectified , correct ? . 
>Shouldn't   this happen first ? 

you can't rectify until you register (by register I mean use i.points
to build a text file that maps reference points in xy coords to real
world coordinates, thats all i.points does) perhaps you understand
something different by "register"


> 3) One final thing can I pull the image directly into a cell directory and 
>    then run r.support on it ? or does i.tape.tm etc do something to the 
>    image ? If so how do I get around this as my data is on CDROM ?

GRASS has a very simple structure and often you can just dump
files into the cell directory and write a header in cellhd.
It depends on the file format of your source. (Have a look
at the programmer manual for info on the GRASS raster file 
structure)

You should be able to run i.tape.tm on a CD rom
as well as tape.  

> 4) Sorry one more thing , when specifying a region I take it that it doesn't 
>    matter what the region is before registration occurs ?

That right, the transformation can be applied to the whole of the images
or to just a smaller region. After using i.points you can run i.rectify
as many times as you like for different regions, resolutions and images
within the group.

> 
> Any comments would be most helpful .
> 
> Thank you 
> 
> Jeremy 
> 
> 
> 
cheers  Tom


Tom Charnock                       O--O
Dept Civil Engineering            (~~~~)
Aston University                 (  __  )
Birmingham B4 7ET UK             /|\  /|\
charnotw at sun.aston.ac.uk