What is threshold elevation in aviation

In aviation, the "Threshold elevation is the highest evelation in the touchdown zone."

See Section 6 a. in the following link:
http://rgl.faa.gov/regulatory_and_guidance_library/rgorders.nsf/0/2635e2fe159b48a286256f1c00699d59/$FILE/order8260.23.pdf

http://pilotsweb.com/pcg/pcg_d.htm
DECISION HEIGHT- With respect to the operation of aircraft, means the height at which a decision must be made during an ILS, MLS, or PAR instrument approach to either continue the approach or to execute a missed approach.
(decision height [DH] is referenced to the threshold elevation).
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WaterStreet is the closest. But Threshold elevation is NOT the highest elevation in the touchdown zone.

The threshold itself is merely the leading edge of the runway you are landing on. The "touchdown zone" is the first 1000 feet of the runway.

So, for example, on approach plates, you'll see a figure like TDZE 423 (for KMYF ILS 28R)   This means that the Touchdown Zone Elevation is 423.. that's that highest elevation in the first 1000 feet of runway.

But the threshold elevation is simply the elevation above sea level (MSL) of the threshold itself. It is NOT the TDZE. If  you look at section 6b of the link that macqueraid provides, you'll see they refer to an example where such is not the case.

Now, there is also a TCH or Threshold Crossing Height. This is the altitude you would be crossing the threshold at if you exactly on glideslope.  Using KMYF again as an example, the TCH for ILS 28R is 41 feet. This is also listed right on the approach plate just like TDZE.

Nickq5 mentions DH or Decision Height. This is NOT threshold elevation, although it is related to it, since DH is usually 200 feet above the threshold, and about a 1/2 mile away.

Since airports are usually very flat (it's not good to take off uphill or land downhill) the TDZE will be usually the same as the threshold elevation, but of course, not always.

you can get any approach plates from the FAA website.

I don't think masqueraid's answer is anywhere near the subject. He might be referring to what we call a Visual Descent Point (VDP) (see http://en.wikipedia.org/wiki/Missed_approach_point).  Even the statement " The problem is these are calculated by survey and not entirely accurate" is ridiculous since a physical survey is by far the most accurate measurement we have available.

A VDP is sometimes recommended during a non-precision approach because the MDA (minimum descent altitude) is often fairly high when you arrive at the MAP (missed approach point).  For example, the MAP could be right over the threshold at an altitude way too high to land.

For example, at KMYF, the MDA is 800 feet on the LOC 28R approach, and the MAP is at DME of 800 ft MSL or about 377 feet. It would be very hard to land on MYF's 3400' runway if you are at this altitude over the threshold.

If you happen to be doing a circling approach, like at KOKB, that's not a problem. The MDA there is 1140 feet, but that's a circling approach so you'll be able to maneuver into a normal landing position.

All of this said, I don't really hear the term "threshold elevation" a lot. Maybe this is more common a term for foreign pilots?  In the U.S. you see TDZE on the charts and TCH but not threshold elevation....

so what is the approach altitude (called) at which the pilot must make his decision?
I'm too high to land now, I won't have enough runway. I need to circle and come in lower.
or
I'm too low..........and the time and distance to regain the proper altitude leaves me too little runway so I need to circle and try again.

Do we passengers assume auto pilot knows the threshold elevation, but if the most dangerous parts of a flight are the takeoff and landing, auto pilot should be in the 3rd seat behind the pilot and co-pilot.

The point at which the pilot must make his decision, on instrument approaches, is called the Missed Approach Point or MAP.  The MAP is always a specific distance from the runway threshold, and at a specific altitude.  This is the FINAL absolute LAST point at which a decision to land or to go missed can be made.  (Many pilots don't wait until this point, and execute a missed approach a little earlier)

On a precision approach, say an ILS (Instrument Landing System) this MAP is coincident with the DH or Decision Height.  It's called a Decision HEIGHT because you are on an electronically guided glideslope. Just follow the slope and when you are a certain specific altitude, THAT is when you "decide."

DH's always are designed to allow you to make the runway using normal landing maneuvers.

By contrast, the other major type of approach is a "non-precision" approach. Here, you don't have a specific glideslope to follow. You have really no vertical guidance. You are just given altitudes that you can't go below at certain points along the path to the runway.

The final, lowest altitude that you can't go below is the MDA, or Minimum Descent Altitude. You have to stay at the MDA (or higher) until you see the airport environment (the runway, approach lights, etc).  If by the time you get to the MAP you don't see the airport environment you legally have to execute a missed approach procedure.

Because of the lack of vertical precision, MDAs are usually much higher than DHs.  When you are at DH, the approach is designed so that you can ALWAYS make a normal, straight-in landing.  But at the higher MDA, with very often at an MAP that is very close to (or right on top of) the airport, you can easily be much too high to land.  You will never be TOO LOW to land (as was your question) but you can easily be too high to land.

And legally, you are "too high" if you cannot land using "normal" maneuvers.  That's why this is this kind-of unofficial thing called a VDP, or visual descent point. This is charted on Jeppesen charts but not on the gov't charts. It's the point at which a normal straight-in landing can be made, from the MDA altitude.

As passengers, you don't need to worry about autopilots. This is all very basic stuff to all instrument rated pilots.  Since autopilots are disengaged before landing anyway (all except for Cat III landings, and most pilots don't trust those) the pilots are manually flying the approach most of the way anyway.