how can i make the data returned from javascript to asp.net persist?

[Opiflux]

I have this page in a wizard where one can select his location on the map.
the value is displayed correctly on a textbox in the same page using:
document.getElementById('MainContent_Wizard1_SelectedLat').value = Lat;
document.getElementById('MainContent_Wizard1_SelectedLng').value = Lng;

till here it's all rosy.
but once i click next in the wizard and try to read the values all i see is the "nothing" I initialized them with.

Is there a way to send back values from JavaScript and catch them in asp.net without losing them?
the code got maimed :/

here it is:
document.getElementById
('MainContent_Wizard1_SelectedLat').value = Lat;
document.getElementById
('MainContent_Wizard1_SelectedLng').value = Lng;