The lateral erosion of the salt marshes in Galveston Island wetlands was investigated through field UAV surveys. Using a GNSS receiver on the ground and a GNSS receiver attached to the UAV, highly accurate location information for the UAV images was derived through PPK analysis. Based on the collected UAV images with location information, Structure from Motion (SfM) by GNSS-supported aerial triangulation was conducted to generate orthomosaic images of wetlands. Drone surveys have been conducted seven times over a year, and the boundary erosion (represented by boundary retreat) for each period was calculated. The averaged error was less than 2 cm for the horizontal direction in the final orthomosaic images of wetlands. The average wave power, hypothesized to be a significant indicator of potential wetlands erosion, was calculated by investigating the wave climate of the survey area through a simplified numerical model. The relationship between wave power and erosion rate was then summarized. The average amount of erosion from 2019 DEC to 2020 MAR, where winds from north to south were dominant, was higher than from 2020 MAR to 2020 OCT. Much erosion was observed in a protruding part of the wetland. This is caused by waves coming from many different directions surrounding these parts. The erosion of this protrusion had a significant influence on the average erosion rate of the survey areas.