GPS Altitude Mystery at Sea Level - Solved?

[BoanviaFx]

I'm a programmer/electronics student and I recently bought a GPS which I hooked up serially with my laptop. I didn't get into much detail of encoding the string which is output from the TX/RX pins of the GPS so I downloaded, a ready made program called "ublox 6". Longitude and latitude are accurate when compared to google maps, but the altitude measured 40m. My question is, isn't the altitude suppose to be 0m at sea level? I was next to the docks, you can confirm using these readings of my location.

Longitude : 14.496701
Latitude : 35.901414
Altitude : 41.300m

Thanks

 

[davenn]

Longitude : 14.496701
Latitude : 35.901414

it mite help if you told us if they were N or S and E or W !

putting them into google Earth as they are put me in the middle of the desert in NE Africa

 

[willem2]

Longitude : 14.496701 E
Latitude : 35.901414 N

Is a location right at the waterline in the harbour of Valetta, Malta.
There's no way that can be 40m above sea level

 

[russ_watters]

We had another thread on this recently that you should look for because I put a lot of effort into digging into the details. The short answer is that because the Earth is not perfectly uniform, sea level is not perfectly round either, whereas the GPS model is. Variations in crust thickness cause variations in sea level height that cause the discrepancy. Here's an article with a map of the discrepancy and an example:
http://www.esri.com/news/arcuser/0703/geoid1of3.html

[edit]
Found it: https://www.physicsforums.com/threads/local-gravity-effect-on-msl-and-geoid.778204/#post-4899368

 

[BoanviaFx]

russ_watters, that link you just sent me explains a lot. Hence the readings I obtained. Thank you for your help! And sorry about that daven my bad!

 

[JakeBrodskyPE]

I should also add that while timing errors do not make as much difference for lateral position, the errors for vertical altitude can be quite significant.

 

[BoanviaFx]

Yes I guess when it comes to my self guided quad-copter I'm going to have to set the initial altitude to the reference or maybe use an external altitude sensor.

 

[russ_watters]

I would think that gps apps for aviators would make the correction for you.

...and actually, your phone probably has a barometric altimeter in it anyway.

 

[JakeBrodskyPE]

I would think that gps apps for aviators would make the correction for you.

...and actually, your phone probably has a barometric altimeter in it anyway.

In aviation, we do not use absolute altitude much. We use air pressure, corrected for local barometric readings, while flying below 18000 feet. When flying above 18000 feet, they set the altimeter to 1013 millibars or 29.92 in.Hg. This is referred to as pressure altitude. An altitude reading based upon pressure alone can have errors due to temperature deviations from standard atmosphere.

So when comparing altitude to GPS in flight, there will be errors induced by standard lapse rate deviations. The reason aviators continue to use pressure measurements is because aircraft performance is actually tied quite closely to the concept of pressure altitude. A further correction can be made by introducing temperature corrections as well. This is called density altitude and it is commonly used at high altitude airports. Aircraft landing and take-off distances are directly proportional to it. Climb rate performance is directly affected by it. Engine performance is tied to it. However, in cruising flight, because we can't know the actual temperatures of the air below us, we use pressure measurements and assume a standard lapse rate. Pilots know to reset altimeters frequently when flying from high pressure to low pressure areas, or from warmer to colder air masses. Otherwise they might be closer to terrain than they intend to be.

Thus a GPS reading of absolute altitude is not only inaccurate, but it is also irrelevant in most cases (except possibly for terrain avoidance). And even in the latter case, corrected pressure based altimetry is easier to use (simpler, updates faster, already there, already in use) than a GPS.

(Yes, I've been an instrument rated pilot for many years)

 

[BoanviaFx]

Thanks for such an informative post! Pressure sensors do seem ideal for aviation rather than gps.