Space Viking by H. Beam Piper

The _Lamia_'s detection picked them up as soon as they were out of the last microjump; Trask's gnawing fear that Dunnan might attack in their absence had been groundless. Incredibly, he realized, they had been gone only thirty-odd Galactic Standard days, and in that time Alvyn Karffard had done an incredible amount of work.

He had gotten the spaceport completely cleared of rubble and debris, and he had the woods cleared away from around it and the two tall buildings. The locals called the city Rivvin; a few inscriptions found here and there in it indicated that the original name had been Rivington. He had done considerable mapping, in some detail of the continent on which it was located and, in general, of the rest of the planet. And he had established friendly relations with the people of Tradetown and made friends with their king.