R help - Jul 2006 - how to make a contour plot using data in long format

Hi List,

There is a dataset with Latitude Longitude information and corresponding 
value shaped like this:
Lati                     Longi                  Value
18.0001             -159.6667             123
18.0023             -159.6665             124
19.1001             -158.2988             125
...                      ...                        ...

I want to use this data to make a filled contour map imposed on an USA map. 
I notice filled.contour command can be used to make contour plots, but it 
requires the data in a matrix. Would someone please tell me if 
filled.contour is the right thing to look at, if so, how to reshape the data 
into a matrix since my Latitude and Longitude are not evenly distributed?

Thanks,
Maria

Maria Wang <mariayzw <at> hotmail.com> writes:
> 
> There is a dataset with Latitude Longitude information and corresponding 
> value shaped like this:
> Lati                     Longi                  Value
> 18.0001             -159.6667             123
> 18.0023             -159.6665             124
> I want to use this data to make a filled contour map imposed on an USA map.
....

Best use package geoR. There are also some examples in MASS (chapter 14).

Dieter