[GRASSLIST:1836] Re: i.ortho.photo on osx

Markus Neteler neteler at itc.it
Thu Nov 20 06:10:35 EST 2003

On Wed, Nov 19, 2003 at 05:44:38PM -0800, Ian Macmillan wrote:
> Richard, 
> So I am amazed, it worked like a charm.  What is more, it also worked much
> faster than using meters (~4 hours processor time with meters, and ~10 minutes
> time with feet).  I am a little confused why this would work though, all you
> are doing is essentially adding vertical exagerration (3.2808 times).  It seems
> like multiplying by any random number would work (e.g. 10, 5, etc.)  Feet makes
> sense only because it is easy to remember.  This goes against an earlier post
> that said big elevation differences don't work that well.  Any ideas from the
> gurus why this would happen?  

This is quite crazy! Does anyone have an idea? The code in question
should be

Does it suggest some precision problems/variable overflows/underflows?

> My next step is to make the DEM file in feet first, then use that as the target
> elevation file originally, and go from there.  This would save the tedious step
> of having to convert everything into feet.
> The only addition I would add to your directions are to make sure that you
> change the target elevation file to the new dem_feet file (obviously).
> Thanks a lot for you help,
> Ian
> Quoting Richard Greenwood <Rich at GreenwoodMap.com>:
> > At 10:04 AM 11/18/2003, you wrote:
> > >Richard, thanks for your response. If you had a similar experience, I am 
> > >pretty
> > >curious to know what happened.  I am not sure that I have a feet to meters
> > >problem though.  I am projecting from an XY location into an UTM
> > projection.
> > >The dem elevation units are in meters.  I projected my dem from a lat-long
> > >location, however the elevation units were meters in that too.  If it isn't
> > >much trouble, I would be curious to see what your case is.
> > 
> > I am by no means an expert with i.ortho.photo. I have only done two 
> > projects with it. But I did just revisit the project that gave me some 
> > difficulty. Initially I was in UTM meters, with a DEM in meters and control 
> > points in meters, but I was getting no joy. When I converted the DEM and 
> > the control points to feet it worked correctly. The project fell by the 
> > wayside, and I had forgotten about it until I read your post.
> > 
> > At the risk of sending you on a wild goose chase, you can test my premise 
> > fairly easily:
> >    1. convert your DEM to feet with r.mapcalc dem_feet = dem * 3.2808
> >    2. convert your CONTROL_POINTS file to feet. (save a copy of your 
> > existing file then convert the elev field for each row to feet.
> > Everything else can stay in meters.
> > 
> > Please keep me posted if you choose to try this.
> > 
> > Rich
> > 
> > 
> > Richard W. Greenwood, PLS
> > Greenwood Mapping, Inc.
> > Rich <at> GreenwoodMap <dot> com
> > (307) 733-0203
> > http://www.GreenwoodMap.com 
> > 
> -----------------------------------------------------
> Ian MacMillan
> Geological Sciences-UCSB

Markus Neteler     <neteler at itc.it>       http://mpa.itc.it
ITC-irst -  Centro per la Ricerca Scientifica e Tecnologica
MPBA - Predictive Models for Biol. & Environ. Data Analysis
Via Sommarive, 18        -       38050 Povo (Trento), Italy

More information about the grass-user mailing list