# carries out central differencing on track data
cendiff = function(spframes, coeffs=c(0.5, 0, -.5)) 
{ 
times = tracktimes(spframes) 
result = filter(spframes, coeffs)
temp = is.na(result) 
result = cbind(result[!temp])
rownames(result) = times[!temp] 
result 
}