Extracting GeoTIFF data with coordinates using R?

I have downloaded the following data as a GeoTIFF and I'm interested in determining the value from this raster at specific locations.

raster = readGDAL("filename.TIFF")
summary(raster)
Object of class SpatialGridDataFrame
Coordinates:
   min max
x -180 180

y  -90  90
Is projected: FALSE 
proj4string :
[+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0]
Grid attributes:
  cellcentre.offset cellsize cells.dim
x            -179.5        1       360
y             -89.5        1       180
Data attributes:
     band1      

 Min.   :  1.0  
 1st Qu.:255.0  
 Median :255.0  
 Mean   :226.9  
 3rd Qu.:255.0  
 Max.   :255.0  
 NA's   :245    

head(data[7:8]) #Xp = longitude, Yp = latitude
              Xp        Yp

    7  -50.41777   1.33138
    8  -59.25075 -19.16780
    9  -67.25043 -27.83412
    10 -54.91759 -15.00130
    12 -67.58375 -28.00078
    13 -65.08385 -34.33386

However, when I run the following, I get an error message:

sp <- SpatialPoints(data[7:8])
summary(sp)

Object of class SpatialPoints
Coordinates:
         min       max
Xp -76.08341 -35.25171
Yp -38.66702   1.33138
Is projected: NA 
proj4string : [NA]
Number of points: 184

data$ndvi<-extract(raster, sp)

    Error in .subset(x, j) : invalid subscript type 'S4'

If I use simulated data for the raster, I don't seem to have the same issue:

simr <- raster(ncol=36, nrow=18)
simr[] <- 1:ncell(simr)
simndvi<-extract(raster, sp)
head(simndvi)
[1] 301 373 408 373 408 444

How can I figure out why this isn't working for the GeoTIFF?

Answer

You are using readGDAL which returns a SpatialGridDataFrame - instead use the raster function from the raster package to create a Raster object. Then extract should work.