Dry ground is no problem. Just make a wall around the footprint with cardboard or plastic sheet and put on a releasing agent. Liquid soap can be used for that purpose. Then pour in your liquid plaster and wait for it to set, which takes half an hour or so.
Snow print is tricky. The heat of the plaster and the weight will likely destroy the footprint. You'll have to spray on several wax layers before applying the molding material. These are specialised sprays that you can buy, although they are a bit expensive.
Before attempting to cast, take photos first from different angles and have a ruler next to the prints for scale. If possible, set up lights on the side (not overhead) to make the details show up better. You'll have something in case the casting process failed and ruined the print.
Snow print is tricky. The heat of the plaster and the weight will likely destroy the footprint. You'll have to spray on several wax layers before applying the molding material. These are specialised sprays that you can buy, although they are a bit expensive.
Before attempting to cast, take photos first from different angles and have a ruler next to the prints for scale. If possible, set up lights on the side (not overhead) to make the details show up better. You'll have something in case the casting process failed and ruined the print.
