[R] How to find data in a map according to coordinates?

[Rui Barradas]

Hello,

You have three coordinates but the problem description only envolves two 
of them, x and y. The code below is valid for any number of dimensions.


min.dist <- function(p, coord){
     which.min( colSums((t(coord) - p)^2) )
}

# Use set.seed to have reproducible simulations
set.seed(1)
test <- data.frame(x=c(1:10),y=c(41:50),temperature=rnorm(1:10))

imin <- min.dist(c(4, 46), test[, 1:2])
test[imin, "temperature"]

Hope this helps

Rui Barradas

Em 09-08-2012 07:56, jeff6868 escreveu:
> Hello,
>
> I have created a spatial map of temperature over an area thanks to
> interpolation. So I have temperature data everywhere on my map.
> My question is: how can I find temperature data on my map for a specific
> location according to coordinates?
>
> For this, I have a data frame containing 4 columns: "x" for longitude, "y"
> for latitude, "z" for altitude" and "temperature" for my data, for each
> pixel of my map. My real data has more than 9 million of rows (because I
> have a temperature data for each 0.0008° of longitude or latitude).
>
> Let's take a smallest example with 10 rows and so just 1° of LAT and LON for
> each pixel (just with "x", "y" and my data):
>
> test <- data.frame(x=c(1:10),y=c(41:50),temperature=rnorm(1:10))
>
> In this example, I would like to ask the user of the algorithm to type on R
> the coordinates (x and y) for which the user wants the temperature.
>
> For example:
> cat("choose your coordinates:\n")
> x: "HERE THE USER SHOULD TYPE A NUMERIC VALUE"
> y: "HERE THE USER SHOULD TYPE A NUMERIC VALUE"
>
> and then R gives the value for temperature (in my third column).
>
> In this example, if the user type 6 for "x" and  "46" for y, R should give
> as a result:   "0.9713693"
>
> And if the coordinates typed by the user are between the coordinates in my
> data.frame, it should response the temperature value of the nearest pixel.
>
> For example, if the user type 3.89 for "x" and 43.62 for "y", R should give
> as a result:   "0.6871172" (value of the nearest pixel: 4 for "x" and 44 for
> "y").
>
> I absolutely don't know how to do this, and if it's easy to do or not. I
> didn't find anything about it on the web for R.
> Have you any idea or suggestions about a package, function or a way to do
> this?
> Hope you've understood!
>
> thanks everybody!
>
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> --
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>
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