use selected-at to extract the altitude from a geopoint (in the form lat long altitude accuracy)
selected at : https://docs.getodk.org/form-operators-functions/#select-questions
convert string: https://docs.getodk.org/form-operators-functions/#number-handling
it would be selected-at(${geopoint}, 2), probably need to convert to a number as well, so adding a fixed value would require a calculate like 500 + number(selected-at(${geopoint}, 2)
extracting values from a geopoint is covered here also