# GPS accuracy

### What is the GPS accuracy and how is this calculated?

The location of the Spider is determined by a U-Blox GPS antenna and module inside the hardware. This calculates the position via the GPS network every second. When a position report is to be sent it will use the latest location information for that position report.

The Spider GPS altitude is Above Mean Sea Level (AMSL). This means you may see a difference between pressure altitude sensors or Above Ground Level (AGL) altitude sensors. The vertical accuracy is approximately within 100 feet. This is in ideal conditions and depends on where the Spider is in relation to the satellites therefore actual position may be affected.

The Speed obtained is instantaneous ground speed using GPS, the GPS module reports a Horizontal Dilution of Position (HDOP) value that is an accurate estimation of the horizontal position. The Spider does not send a position report with an HDOP value of more than 8. The horizontal accuracy is approximately within 10 feet. This is in ideal conditions and depends on where the Spider is in relation to the satellites therefore actual position may be affected.

### GPS Altitude vs Barometric Altitude

We often have questions about the difference between the reported barometric altitude and the Spider’s GPS altitude. These are two very different parameters, one based on atmospheric pressure and one based on a geometrical calculation. So there can be differences between the two parameters.
The Spider reports GPS altitude. GPS altitude is calculated through a satellite fix that determines the height above mean sea level, it is not related to atmospheric pressure or RADAR altitude, it is based on a geometric calculation of GPS satellites.

The barometric altimeter calculates pressure altitude based upon the atmospheric pressure setting applied to the aircraft’s altimeter (QNH/QFE).

The two calculations are therefore based upon fundamentally different parameters:
• GPS altitude a geometrical calculation.
• Barometric altitude a pressure-related calculation.
So there can be differences when GPS altitude and pressure altitude are compared.

### Spider GPS Altitude Post Flight Analysis

While there can be differences between GPS altitude and Barometric altitude, it should be noted that the Spidertracks tool is still a valuable source of post-flight information.  An excellent way of comparing flight paths is to use the export to KML/Google Earth Feature.  This allows a historical track to be taken, downloaded as a KML, and then displayed in Google Earth, providing a great reference picture of where the aircraft was in terms of the local terrain.
FAQs
Q. It appears that the altitude values provided from the Spider8 and Spider6 are just the raw GPS altitude (converted to feet) - can you confirm whether the number has been offset by the Geoid Separation value (for example field 12 in \$GPGGA NMEA sentence)?
A. The Spider 6/8 devices report the altitude value from the 10th field of the GGA sentence, which should be a mean sea level (geoid-based) value. We do not use the geoid separation value at all.