Jérôme,
You can disable this using a define on a StateSet in your code.
For a specific layer:
layer->getOrCreateStateSet()->setDefine(
"OE_TERRAIN_RENDER_ELEVATION",
osg::StateAttribute::OFF);
For the whole map, you need an override:
mapNode->getOrCreateStateSet()->setDefine(
"OE_TERRAIN_RENDER_ELEVATION",
osg::StateAttribute::OFF | osg::StateAttribute::OVERRIDE);
Hope this helps.
Glenn Waldron / Pelican Mapping