Smartphones are great ways to connect to the Internet, use the data with an never-ending variety of apps that use that connection for different purposes. But the convenience of having Internet connection doesn't go well with current data plans offered by mobile carriers.
Data caps limit what we can consume and some features found on apps make it difficult to not go over those caps. One such thing are location services in the background.
Some apps use this feature to update your location, even when you are not using the app. GPS, Wi-fi and even cellular radios are used to update your current location. Cellular radios (Significant-Change Location Service) are used as a low power alternative to using GPS, which often drains the battery faster.
Apps that need to transfer data between the service and the app will consume your data. The iOS device will continuously pin cell towers to get your current location.
Recently I experienced this the last month and I only noticed when I saw a spike in my data use. I used two apps with background location that continuously transfer data between the service and the app: Saga and Dark Sky.
Many people don't realize that actual data is transferred while location services are working in the background.
According to Dark Sky's FAQ regarding location services:
"When Dark Sky is closed, we don't use your device's power-hungry GPS. Instead, we rely on battery-friendly cell tower triangulation, which means you shouldn't notice any effect on your battery life."
Again, that's great for the battery life, but not that good for your data plan. This also means that apps that use cell radios to update location won't play nicely for Wi-Fi only iOS devices like the iPod Touch and Wi-Fi iPad.
Although location services in the background is a great feature to get updated information regarding your location like in Dark Sky's case if it's going to rain where you are, the data transfers that take place can burn you data plan faster, not all, but faster.
How do you now if an app is currently using location in the background? The location icon in the status bar is continuously there and is represented by a purple arrow in the location services settings. To turn it off head to Settings > Location Services and you will see all apps that have permissions to run location services. The one with a purple arrow nest to it is the one using location on the background. Turn the switch off to turn the feature off.