Converting from ellipsoidal to arithometric heights (egsa 87)

[BarbaraBerna]

I try to conver from ellipsoidal to orthometric heights with arithmetic function. In input raster 2 i put WGS84.img from \peodata\geoid folder.The results are not right. And now i have the question. The input raster 2 is always the same or it is depended on coordinate system? my files which i want to convert are EGSA87/TM87. What i must put in raster 2=N to convert from ellipsoidal to orthometric in EGSA87?

[MelitaKennedy]

Did the height value change at all?

I do not work with rasters very often. I think you may need to put one into the other's coordinate system, including applying the geographic/datum transformation, GGRS_1987_To_WGS_1984_1. Even if the software is converting one raster to the other coordinate system internally (on-the-fly) before doing the calculation, the datum transformation may not be done, which would throw off the results.

Melita