# [Proj] How to offset height above ellipsoid.

Paul Chavent paul.chavent at fnac.net
Mon Mar 23 17:14:26 EST 2015

```Hi.

I try to convert lat/long coordinates to eqc. The origin of the eqc projection plane is at (43.315959 1.402896 320.000). My eqc specification take into account the altitude of the projection plane, so the plane should be tangeant to a sphere with a radius of 6368437.553703493.

The specification of the translation is then :

+proj=longlat +datum=WGS84 +no_defs +to +proj=eqc +a=6368437.553703493 +lat_ts=43.315959 +lat_0=43.315959 +lon_0=1.402896 +no_defs

At this point, if i give an altitude in the LLA input, it isn't translated since no datum have been specified for my destination projection.

For datum conversion, +towgs84 and +nadgrids seems to be the preferred choice for horizontal datum shifting, although +geoidgrid seems the preferred way for vertical datum shift.

In order to add a constant offset to the LLA altitude, i could provide a "gtx" file with only one value for the whole lat/long space.

I wonder if it is a good practice, or if there is other way of doing it ?

Wouldn't be possible to specify the offset on the description string for such a simple case (that also would be easier in the case of the usage of the c API with frequent altitude offset changes) ?

The expected result is

cs2cs -rs \
+proj=longlat \
+datum=WGS84 \
+no_defs \
+to \
+proj=eqc \
+a=6368437.553703493 \
+lat_ts=43.3159590 \
+lat_0=43.3159590 \
+lon_0=1.4028960 \
+no_defs \
<<EOF
43.3159590  1.4028960  320
43.3155991  1.4024014  340
43.3155991  1.4033906  360
43.3163189  1.4033906  380
43.3163189  1.4024014  300
EOF
0     0   0
-40   -40   20
-40    40   40
40    40   60
40   -40  -20