Collect each map before digging up its treasure. I would do this after the story, with both arrow types unlocked and the lake at its lowest level. Complete the dwarven favors in Like Oil and Water to open the mines, storeroom and Northri routes.
Open Goals to read the maps you own. If a dig spot offers no interaction, check that you actually picked up its map. All twelve digs are required. The pairs below give a starting landmark for both pickups.
Dead and Bloated
Find the map: During Second Hand Soul in Volunder Mines, look for the rune-locked gate before the Soul Eater area. Descend the nearby chain and search the floor beside the red chest.
Dig: At Cliffs of the Raven, reach the dead giant at the back of the beach. Search the sand between its arm and leg.
Kneel Before Thor!
Find the map: Enter Landsuther Mines for Deus Ex Malachite. Near the beginning, break through the floor and descend into the lower room. Take the side elevator on the left to the ledge with a red chest. The scroll is beside it.
Dig: Climb Lookout Tower to its upper path and use the zipline toward Muspelheim Tower. Search immediately to the right of the wooden Jötnar shrine.
Don't Blink
Find the map: During Fafnir's Hoard, reach the circular central chamber in Fafnir's Storeroom. Search beside the corpse on the central platform.
Dig: Return to the large carved face at the Mountain entrance. Search the ground beneath its nose, just outside the doorway.
Njörd's Oarsmen
Find the map: During or after Family Business, dock beside Northri Stronghold's raider ship. Smash the wooden boards in its hull and enter the lower deck. The map is beside a corpse.
Dig: From the southern Mason's Channel landing, climb toward the Council of Valkyries. At Sindri's shop, climb the wall opposite, then take the right-hand recess toward the giant oarsman statue.
Creation Island
Find the map: After the lake's second water drop, land on Iron Cove's southern beach. Look on the ground directly ahead of the landing.
Dig: Circle to Buri's Storeroom's northeastern shore. Search between the rocks near the waterwheel, facing the large gate toward Veithurgard.
Finder's Fee
Find the map: Land at the Forgotten Caverns' southern beach and enter the cave beyond the spirit. Check the back-left corner beside the chest and Mystic Gateway.
Dig: In Northri Stronghold's waterwheel area, dock and climb the wooden tower beside the landing. Search the ground at the top near the corpse.
The Boat Captain's Key
Find the map: At Stone Falls, climb to the spirit and the small drawbridge. Break the bridge's catch with an axe throw, cross it and pick up the scroll on the ground.
Dig: Climb to the top of Isle of Death. Enter the stranded shipwreck and look for the digging spot beneath its hull.
Island of Light
Find the map: At the southern Mason's Channel landing, search the sand beside your boat before climbing away from the beach.
Dig: At Light Elf Outpost, create the light bridges and climb past the Nornir chest. The spot is behind the locked gate on the outer path. Continue uphill, open the upper gate with its chain and carry the loose blue crystal from near the Nornir chest through it. Cross the light bridge and place the crystal in the next socket to extend the route. Follow that path and descend the chain. Search to your right, then open the gate for a shortcut.
The Historian
Find the map: Dock at Ruins of the Ancient in the northwest lake. Walk between the two pillars just inland and collect the scroll from the sand.
Dig: In the woodland outside Fafnir's Storeroom, approach the large entrance with two animal-head statues. Dig beneath the statue on the left.
Hunter's Kingdom
Find the map: Dock at Lookout Tower and search the ground by the Nornir chest. If the water is low, climb back up to this earlier shoreline.
Dig: Enter Veithurgard through the water gate opened at Stone Falls. Continue past the dragon toward the fortress's four-rune door. Search beside the lore marker and broken statue to the left of that door.
The Last Place They'd Look
Find the map: From the Summit gateway, follow the snowy pass away from Mimir's tree. Take the climb-down on the left into the lower passage with Sindri's shop. Pass the shop, squeeze along the wall and search the tunnel floor beyond.
Dig: Reach the Council of Valkyries' circle of thrones. Search the left side of the arena beneath the face carved into the cliff.
The Turtle's Tribute
Find the map: Use the Witch's Cave Lower Level gateway and ride the lift. Activate the blue crystals to cross the cave. Beyond the Nornir chest, throw a red sap crystal at the sap barrier and detonate it with a shock arrow. Collect the scroll on the floor before the Hidden Chamber.
Dig: Return to the lift, clear the sap behind it and climb the well to the garden. Search beside the turtle house's front left foot.