Local Coordinates for ecef2lla Function

Hello everyone,

I have some X, Y and Z coordinates in an ECEF Reference System, coming from a GPS receiver. When converting the coordinates with the function ECEF2LLA, I would like to get more precision (I'm having an offset of about 20 km now) in calculating the position by specifying the flattening and the axis length of a local ellipsoid. Is there a way/tool to get those parameters by knowing the latitude and the longitude of a reference point on Earth? Also, when calling the function I get a third parameter (altitude) which is of the order of 10e8: shouldn't it be the distance from Mean Sea Level, and so, way much lower?

ecefX = 445907859;          ecefY = 69298199;          ecefZ = 449249485;

LLA = ecef2lla([ecefX, ecefY, ecefZ], 'WGS84')

The output I get is this below. Latutide and longitude as I said are not completely exact and the altitude should be around 83 meters.

1.0e+08 *

   0.000000448739703   0.000000088336289   6.303908195856305