A land surveyor is a professional who marks boundaries on a property, and who also studies the topography of a piece of property, including natural features, drainage issues, and the like. A homeowner may not need to call a land surveyor very often, but their services can be invaluable for a number of reasons; note a few of those reasons here, so you know when you might want to call land surveyors to your property, and why their services can be invaluable.
Any property improvements
Obviously you should know the actual, legal boundary of your property before you install a new fence, but it's good to order a land survey before you perform any type of exterior property improvements. This includes adding landscaping trees, erecting an outbuilding such as a shed, barn, garage or silo, and even adding a small structure such as a pergola. Having a survey done before you erect any such structure will ensure you are not encroaching on a neighbour's property and will not risk damaging a neighbour's property by creating drainage issues.
If you're thinking of dividing and selling a parcel of your property, you want to ensure you call for a land survey first. Dividing or parcelling your property may actually interfere with a neighbour's access to their property, and you may then need to include an easement on that divided land. This might reduce that divided property's value, and an easement would also need to be considered for any construction projects planned for that parcel. Once the survey is done, you might want to adjust your plans for this parcelling, or adjust your asking price for that divided property.
If you've found that your neighbours have made improvements to their property and that those changes are interfering with your property, you may want to call a surveyor before confronting your neighbours. It may be that you're mistaken about where your property boundaries fall, and you may not have access to certain areas of land that you assumed. On the other hand, if your neighbours are encroaching on your property or have improperly blocked access to your own property, you may have legal rights for an easement or other such modifications to their property. After a property survey is completed, you can then consult with an attorney about your rights for accessing your property, or for ensuring that a neighbour's outbuilding, trees, and the like are not interfering with your property.