Horizon Explorer requires ARCore (Android) or ARKit (iOS) technology to implement the Augmented Reality view.
Apple devices from the iPhone 6S with iOS 11.0 or later will support ARKit & Horizon Explorer.
The full list of Android devices is available from Google here: ARCore Supported Devices.
If you cannot don't see Horizon Explorer in your app store, you are probably not using a supported device.
There are 2 things you can try to improve accuracy:
1) Calibrate your phone's compass by moving it in a figure-8 shape, then pressing "Auto Align".
2) Drag the white horizon line with your finger so that it matches up with the horizon line you can see through the camera.
Press the blue cog/gear button to open the settings menu. You can choose your units of measurement from the dropdown menu.
If you have a worldwide open dataset which you think would improve Horizon Explorer, please email details to hello@horizon-explorer.
If you want a similar app to Horizon Explorer which displays your own data for internal use, you can email hello@horizon-explorer to discuss licensing & pricing.