When you are at your base, you don't realy claim land but improve the defence of your base.
When you do this a text pops up saying "Success! Your presence on a base increased it defences by 2 points for 2 hours! Check in within that time again for an increased effect and duration!"
So you use the "capture land" button within the range of your base to "check in".
That is basically the idea of any AR-game. You have to move and go outside to play the game. Therefore, you are only able to interact with your nearest peace of land which has to be not more than 500 meters away from you. If you want to capture new land, build buildings on your land or level land up, you always have to go there (in range of 500 meters)
You are always able to click on your land and then on the upcomming panel to show surrounding things you might not be able to see from your location. You can also always collect resources that spawned on your land with the tracking sattelite no matter where you are.
whooops @Mr. D was faster and more precisely of course