Convert coordinates from WGS 84 (EPSG:4979) to WGS 84 + EGM2008 height (EPSG:9518) — official EPSG transformation parameters, accuracy and area of use, with a free in-browser converter.
The EPSG Geodetic Parameter Dataset defines 2 transformations between WGS 84 (EPSG:4979) and WGS 84 + EGM2008 height (EPSG:9518). Each applies to a specific area — pick the one whose area of use covers your data.
Grid file: Und_min1x1_egm2008_isw=82_WGS84_TideFree
Reversible alternative to WGS 84 to EGM2008 height (2) (code 3859).
Grid file: Und_min2.5x2.5_egm2008_isw=82_WGS84_TideFree
Reversible alternative to WGS 84 to EGM2008 height (1) (code 3858). Grid spacing is 2.5 arc-minutes. For a file with smaller (1 arc-minute) grid spacing (in principle more exactly representing the spherical harmonics) see CT code 9618.
The same transformation with common GIS tools and libraries:
from pyproj import Transformer
transformer = Transformer.from_crs("EPSG:4979", "EPSG:9518", always_xy=True)
x, y = transformer.transform(-73.9857, 40.7484)
print(x, y)geographic system using latitude, longitude and ellipsoidal height · Area of use: World
+proj=longlat +datum=WGS84 +no_defs +type=crsMore about EPSG:4979 →
compound horizontal + vertical reference system · Area of use: World
+proj=longlat +datum=WGS84 +geoidgrids=us_nga_egm08_25.tif +geoid_crs=WGS84 +vunits=m +no_defs +type=crsMore about EPSG:9518 →