diff --git a/models/transformation.py b/models/transformation.py
new file mode 100644
index 0000000..dd836fb
--- /dev/null
+++ b/models/transformation.py
@@ -0,0 +1,39 @@
+import numpy as np
+
+
+def visible_cells(h):  # calculate visible ERA5 surface cells (30 km x 30 km) as function of balloon height
+    if h < 0:
+        res = 4
+    elif h > 40000:
+        res = 2601
+    else:
+        res = h * 2597/40000 + 4
+    return res
+
+
+def transform(lon, lat):  # longitude and latitude (in degree) to x, y, z (in m) in WGS84 coordinate system
+    # WGS 84 reference coordinate system parameters
+    a = 6378137.0  # major axis [m]
+    e2 = 6.69437999014e-3  # eccentricity squared
+
+    lon_rad = np.radians(lon)
+    lat_rad = np.radians(lat)
+    # convert to cartesian coordinates
+    r_n = a / (np.sqrt(1 - e2 * (np.sin(lat_rad) ** 2)))
+    x = r_n * np.cos(lat_rad) * np.cos(lon_rad)
+    y = r_n * np.cos(lat_rad) * np.sin(lon_rad)
+    z = r_n * (1 - e2) * np.sin(lat_rad)
+    return x, y, z
+
+
+def radii(lat, h):  # get radii to transform local velocity vectors in m/s to change in lat, lon (in deg/s)
+    # WGS 84 reference coordinate system parameters
+    a = 6378137.0  # major axis [m]
+    e2 = 6.69437999014e-3  # eccentricity squared
+
+    lat_rad = np.radians(lat)
+    # convert to total distance to center of Earth (used for latitudes)
+    r_lat = h + a / (np.sqrt(1 - e2 * (np.sin(lat_rad) ** 2)))
+    # determine distance to Earth rotation axis (used for longitudes)
+    r_lon = np.cos(lat_rad) * r_lat
+    return r_lon, r_lat