GPS Accuracy Overview

SpeedOdo App


GPS Accuracy

GPS devices work pretty well MOST of the time. But there are always complaints that a GPS device (or software like SpeedOdo) is not accurate. But it may not be the fault of the device (...or software like SpeedOdo). There are many factors that could cause inaccurate GPS readings. The problem is literally a moving target. There could be buildings blocking or redirecting your signal, you could be underground, or there could be a hiccup in the GPS systems themselves.

Pro Tip

Wave your phone or tablet around slowly in a figure eight pattern 3 times. On many devices this will cause the GPS to recalibrate.

Here is a good website with GPS accuracy information by the US government:

See the note below from Apple about "Improve GPS Accuracy", if you are having problems with GPS accuracy.

Apple GPS Info

Per Apple's instructions, this disclaimer must be included with any app that uses the GPS and Location Services:

Continued use of GPS running in the background can dramatically decrease battery life.

So if you plan on using SpeedOdo for a long trip, you should bring whatever you need to charge your iPhone or iPad. (typically this will be your USB cable and a USB DC adapter)

On this page (About Privacy and Location Services), Apple has this to say about improving your GPS accuracy:

Improve GPS accuracy

GPS accuracy depends on the number of visible GPS satellites. Locating all visible satellites can take several minutes, with accuracy gradually increasing over time. To improve GPS accuracy:

  • Make sure that you've set the date, time, and time zone correctly on the device in Settings > General > Date & Time. If possible, use Set Automatically.
  • Keep a clear view in several directions. Walls, vehicle roofs, tall buildings, mountains, and other obstructions can block line of sight to GPS satellites. When this happens, your device uses Wi-Fi or cellular networks to determine your position until the GPS satellites are visible again.

GPS Accuracy

The GPS Accuracy is the horizontal accuracy, and is defined by Apple as:

The radius of uncertainty for the location, measured in meters.

This means that a returned location is certain (accurate) within a circle of this radius. Think of the actual location being within a circle, centered at this location, with a radius of the horizontal accuracy. The smaller the circle (smaller radius), the more certain (accurate) the location. The larger circle (larger radius), the less certain (accurate) the location.

Every time a GPS location is returned, it comes with a horizontal accuracy value. By selecting this option you will always see what the accuracy was for the latest location update.

NOTE: when making the initial GPS connection, the accuracy will be large and should gradually get smaller in a few seconds. This can cause slight fluctuations in the returned speed and distance if you start moving as soon as you start using the GPS. If you wait a couple of seconds (30-60 seconds is a decent estimate) before you actually start moving, the GPS accuracy should settle down to 5 or 10 meters (17 or 33 feet).

Minimum Distance Filter

Changing this from the default (0), will significantly change how Location Services returns information and how SpeedOdo functions.

The Minimum Distance is chosen in the Settings. The default is No Filter (zero).

According to the Apple docs, distance filter, is defined as:

The minimum distance (measured in meters) a device must move horizontally before an update event is generated.

In other words, this means that there will only be location updates when you have gone at least as far as this distance filter. If you set this filter to zero (none), you will get location updates as soon as they are available. (In practice, this is often once every 1-3 seconds).

Setting the minimum distance will alter how and when the trip and total odometers are accumulated.