Altitude Reference: Stratus 2 WAAS or Altimeter?

[designrs]

The Stratus 2 now provides WAAS accuracy. My correctly set altimeter showed about a 250' difference in altitude from the Stratus. So what altitude reference is best?

If I'm flying VFR altitudes (3,500, 4,500 etc.) which altitude reference is better to use, WAAS or altimeter?

What if I'm flying just above Maximun Elevation Figure for a given quadrant? Which altitude reference will keep me safer?

What about flying under a Bravo shelf? What altitude reference is best?

Thanks.

[SportPilot]

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[designrs]

OK. So that's the FAR. Thank you.

Isn't WAAS more accurate?
What do IFR pilots use? I would think that IFR relies on WAAS especially when down to minimums?

[SportPilot]

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[FastEddieB]

SportPilot is right.

We all have to fly indicated altitude, or all bets are off re: separation.

And that will almost always be different from true altitude, given that pressure and temperature lapse rates are rarely standard.

[designrs]

Got it.

I wouldn't be surprised if that FAR doesn't get rewritten in 2020 when supposedly everyone will have WAAS GPS ADS-B. Somehow the whole airspace standard will have to reconcile the two systems. Then again, I suppose RADAR stations have been reporting true altitude all along?

[Merlinspop]

It might be more accurate (I don't know), but not everyone has a GPS, let alone a WAAS GPS. Remember, even after 2020, not everyone will have to have one, depending on where they fly, and there are lots of places where both equipped and non equipped will be in the same airspace.

[FastEddieB]

Only tangentially related, but...

...when told to fly a heading, don't try to show off with your fancy GPS thingee and fly that precise course instead.

The controller knows the prevailing winds, and assigns headings with those already in mind.

I know of several people who were in the habit of doing this, and like altitudes it works best if everyone follows the same conventions.

[BrianL99]

Only tangentially related, but...

...when told to fly a heading, don't try to show off with your fancy GPS thingee and fly that precise course instead.

The controller knows the prevailing winds, and assigns headings with those already in mind.

I know of several people who were in the habit of doing this, and like altitudes it works best if everyone follows the same conventions.

Good & accurate advice, Eddie. We're starting to see more and more pilots who learned with glass and forget there was a whole different world out there, around which the current regulations and customs were designed.

[dstclair]

This questions comes up from time to time. Basically your GPS altitude is more accurate but pressure altitude is what is used to fly in the system.

c162pilot wrote:
Correct me if I am wrong here but I would assume that barometric altitude with the correct pressure setting should equal GPS altitude.

I do not know about the accuracy of the barometer in the iPhone but I have noticed that the GPS altitude can be off by several hundred feet from the indicated altitude (with current baro setting) that I see on my panel also Garmin Pilot has both a psudo panel with a VSI and a PFD with VSI that may be more accurate with a real barometer I would suspect.

My prior answer:

Nope, you would need to correct for temperature as well and the pressure you input will never be precise for your specific location and altitude. The ATIS/ASOS reading is at ground level at a specific point. It can vary quite a bit just a few miles away or even a couple thousand feet up. Really cold or really hot days (big difference from 59F) can introduce an error of a few hundred feet.

BTW -- GPS altitude is likely to be more accurate than your basic altimeter. GPS altitude is based on the WGS-84 ellipsoid model (sometimes called HAE) which can be several hundred feet off depending on the distance from the poles. But many GPS units (and the ADS-B system) map this ellipsoid to a more refined geoid model (GEOID12A) which is highly accurate. With a WAAS GPS your true altitude will be within a couple meters.

Altimeters have an allowable error of 200 ft. Also, temperature deviation from standard further erodes the accuracy by 4% for each 10C delta from ISA.

[CTLSi]

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[CharlieTango]

In an all glass cockpit ... the heading (HSI) is sourced from the GPS input, you know the 'thingee' that guys with only a mag compass don't have...

The heading comes from the magnetometer. GPS can supply your track when you are moving but it doesn't account for crabbing into the wind. When you do so your track and heading do not match.

I have 2 GPS on my panel, one in my EFIS and one in my 496. The EFIS with its magnetometer can depict the plane's crab as well as follow track vs bearing, the 496 can't do that.

[CTLSi]

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[3Dreaming]

CTLSi, do you have a magnetic compass located overhead on the cross tube in your airplane?

[CharlieTango]

When there is no wind, both Track and Heading will be the same.

My CT can't be bothered keeping its nose in the wind. When I focus I focus on matching track and bearing. You can swing your nose left or right, changing your heading while maintaining your track. The same tendency to fly happily while a bit sideways causes uneven fuel usage and erroneous wind calculations too.