What is the depth of the frost line in Prince George's County?

PG County local amendment §4-247
Footings; Section R-403.1.4, Minimum Depth.
Section R-403.1.4 is amended to read as follows: "Minimum Depth." All exterior footings and foundation systems shall extend below the frost line. All exterior footings shall be placed at Thirty (30) inches below the undisturbed ground.

Exception: Frost-protected footings constructed in accordance with Section R-403.3 and footings and foundations erected on solid rock shall not be required to extend below the frost line. In Seismic Design Categories D1 and D2, interior footings supporting bearing or bracing walls and cast monolithically with a slab on grade shall extend to a depth of not less than Eighteen (18) inches below the top of the slab.