Isn't one of the fundamentals of research, comparison?
Sorta kinda but sometimes not really. If you have treatment groups to compare, then yes the comparisons obviously matter. Observational research doesn't necessarilly have to be compared against anything. It's just observations, people can draw comparisons afterwards. But in order to draw comparisons, there's many things to consider. The who, what, when, where, and how are really what is fundamental if you want to make comparisons with observations. More importantly, it must be documented.
In Eagles case, there are comparisons which could be made, but they are dependent on many questions first. But since he has zero documentation (no data), these can't really be answered. Has there been significant erosion? Is the land subsiding or rising? Is there significant depostion of material? Where were the measurements made? How were they made? When were they made? What is the expected sea level rise for Eagle's location? (mean sea level rise doesn't mean we should expect the same amount of sea level rise across the globe) How does the measured change in Eagle's location differ? Is it significant? Has the methodology changed or been consistent through the time series? If so how did it change?
And on, and on. That's what good looks like.