GPS Alimeter

Sterlingtimes

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

born_again

Some use air pressure.
Such as in fitness watches, & stair counting.

MattMattMattUK

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

Yes, but it depends on location and equipment used. To be accurate it requires equipment capable of that, some phones have it, some do not. It also requires visibility of more satellites than is required for location to give an accurate reading. If you are out in fields then it will probably be fairly accurate, if you are between buildings, or up a skyscraper then almost certainly not, if you are flying in a plane then a phone would struggle because it is not designed for that, but a civilian aeroplane's navigation GPS would be accurate to within a few tens of meters vertically. 

Undervalued

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

Short answer - No.

Obviously though that depends on what degree of accuracy is required.

In basic terms, GPS satellites are fairly low in sky to allow accurate triangulation N S E W but the trade off is significantly less accuracy up and down. 

Aircraft altimeters are barometric but the pilot needs to be told the pressure either at sea level (QNH) or at the airfield (QFE) and set that in order for the altimeter to read the height correctly.

Briefly, if the pilot expects to return to the same airfield without landing elsewhere they will probably set the meter to read zero before they take off. If they intend to land elsewhere they will set it to the height of the take off airfield above sea level. Obviously pressure can change so if they are flying for any length of time and are in radio contact with ATC they will be told the current pressure shortly before they prepare to land.

Frozen_up_north

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

There are GPS modules that have very accurate position and altitude capabilities, one such module is the UBlox ZED-X20P. The specification is to an incredible accuracy, mainly for a fixed location and stationary for several hours with aux correction signals. The data sheet is at:

https://www.u-blox.com/sites/default/files/documents/ZED-X20P-00B_DataSheet_UBXDOC-963802114-12690.pdf

wongataa

Frozen_up_north said:

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

There are GPS modules that have very accurate position and altitude capabilities, one such module is the UBlox ZED-X20P. The specification is to an incredible accuracy, mainly for a fixed location and stationary for several hours with aux correction signals. The data sheet is at:

https://www.u-blox.com/sites/default/files/documents/ZED-X20P-00B_DataSheet_UBXDOC-963802114-12690.pdf

GPS by design does not have as good altitude accuracy as horizontal accuracy.  No GPS device can change the accuracy levels built into how the system works.  Generally the little bit worse altitude accuracy doesn't matter.  Any GPS device used with external correction data will give more accurate results.

Undervalued

wongataa said:

Frozen_up_north said:

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

There are GPS modules that have very accurate position and altitude capabilities, one such module is the UBlox ZED-X20P. The specification is to an incredible accuracy, mainly for a fixed location and stationary for several hours with aux correction signals. The data sheet is at:

https://www.u-blox.com/sites/default/files/documents/ZED-X20P-00B_DataSheet_UBXDOC-963802114-12690.pdf

GPS by design does not have as good altitude accuracy as horizontal accuracy.  No GPS device can change the accuracy levels built into how the system works.  Generally the little bit worse altitude accuracy doesn't matter.  Any GPS device used with external correction data will give more accurate results.

It does if you rely on it then fly into the top of a mountain!

Sterlingtimes

MattMattMattUK said:

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

Yes, but it depends on location and equipment used. To be accurate it requires equipment capable of that, some phones have it, some do not. It also requires visibility of more satellites than is required for location to give an accurate reading. If you are out in fields then it will probably be fairly accurate, if you are between buildings, or up a skyscraper then almost certainly not, if you are flying in a plane then a phone would struggle because it is not designed for that, but a civilian aeroplane's navigation GPS would be accurate to within a few tens of meters vertically. 

That's an educated response, and I appreciate it. At least with my 'phone, the map-based assessments appear to be more reliable.

Sterlingtimes

Undervalued said:

In basic terms, GPS satellites are fairly low in sky to allow accurate triangulation N S E W but the trade off is significantly less accuracy up and down. 

Aircraft altimeters are barometric but the pilot needs to be told the pressure either at sea level (QNH) or at the airfield (QFE) and set that in order for the altimeter to read the height correctly.

That is a precise answer to my question. Much better than AI responses. Thank you. 

I am now 70, and I have inherited my father's Edwardian silver altimeter. I was fascinated with it as a child. I would always set it at sea level when we had a day out. We once drove to the summit of Mount Teide in Tenerife, which rises to 3,718 meters (12,198 feet). It is a splendid instrument and surprisingly accurate. I thought that the GPS apps might outperform it in the vertical plane.

Sterlingtimes

wongataa said:

GPS by design does not have as good altitude accuracy as horizontal accuracy.  

I suppose that you have reframed my question. Thank you. It appears that GPS using 'phones is not entirely accurate in three dimensions.

wongataa

Undervalued said:

wongataa said:

Frozen_up_north said:

Sterlingtimes said:

Altimeter apps measure height by reference to a map (location-based) or by GPS alone. The GPS method can provide some very dubious readings. Is GPS alone an accurate method of measuring height above sea level?

There are GPS modules that have very accurate position and altitude capabilities, one such module is the UBlox ZED-X20P. The specification is to an incredible accuracy, mainly for a fixed location and stationary for several hours with aux correction signals. The data sheet is at:

https://www.u-blox.com/sites/default/files/documents/ZED-X20P-00B_DataSheet_UBXDOC-963802114-12690.pdf

GPS by design does not have as good altitude accuracy as horizontal accuracy.  No GPS device can change the accuracy levels built into how the system works.  Generally the little bit worse altitude accuracy doesn't matter.  Any GPS device used with external correction data will give more accurate results.

It does if you rely on it then fly into the top of a mountain!

You shouldn't cutting it that fine with clearance for it to matter.  It is a few meters less accurate in the vertical dimension - not a huge amount.