Survey Markers For The Future

I have a great deal of land that I inherited from my grandmother who had owned a large farm outside of town. The area around her farm is being developed by many different builders and they are interested in the land that I now own. I decided it would be wise to see exactly what there is of the farmstead so I hired a company to come and place very long term survey markers there so that any time we need to check the land limits, they can be read to verify the land is really ours to be sold and developed.