[R-sig-Geo] georeferencing point shape

[Tom Gottfried]

Hi Tim,

> I have a point shape containing field measurements in a small scale studyarea. Unfortunately these
points didn`t have geographic or projected coordinates. The spatial reference for these points are
theodolite measurements. For four corner points gps measurements of projected coordinates are
available. I want to georeference my point shape in these projected coordinate system and have no
idea how.
> Here is a subset of my data:
>
> x <- c(5.786, -3.766, -12.613, -21.836, -26.340, 3.958, -11.120, -17.266, -18.938,
>   16.751, -21.507, 26.420, -19.916, 23.184, 9.660, -5.711, -18.256, 27.888, 12.634, -33.510)
> y <- c(4.470, 0.797, -3.130, -7.656, -9.313, 8.700, 6.923, 8.545, 12.468, 4.748,
>   -20.920, -5.396, -24.830, -11.636, 22.462, 19.210, -28.851, -9.510, 27.421, 8.035)
> z <- c(1100.493, 1100.867, 1101.798, 1102.559, 1103.703, 1102.366, 1107.249, 1110.781,
>  1113.920, 1096.967, 1095.284, 1088.956, 1092.869, 1086.786, 1101.300, 1110.197, NA, NA, NA, NA)
> gps_x <- c(NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA, 4560136, 4560184, 4560172, 4560121)
> gps_y <- c(NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA, 5289465, 5289482, 5289521, 5289502)
>
> d <- data.frame(x,y,z,gps_x,gps_y)
> d
> coordinates(d) = ~x+y
> plot(d, axes=T)
>
> gps_points <- d[17:20,]
> points(gps_points, pch=19, col="red")
>
> Because both measurements - the theodolite values and the gps values - are measured in meters, my
> first idea was to calculate the coordinates for all points by sum the difference between a gps
> point and every single theodolite point in x- and y-direction. Unfortunately there are little
> differences depending which gps point is used as basis.

If you can assume the little differences being due to the error in gps-measurements, you could
calculate your coordinates with all your gps points as a basis and then take the mean as final value.

Tom

> Is there a better way to transform the point pattern into the projected coordinate system?
> The proj4string for my coordinate system is:
> +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel +units=m +no_defs
>
> Thank you very much.
>
> TIM
>
>

-- 
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