In this thread: What type of altimeter is this?

there are 3 images of altimeters that have a pointer that moves around the outside diameter of the scale markings and they have another pointer that moves around the inside diameter of the scale markings, e.g 4 and 5 on this picture.

enter image description here

  • What is the purpose of the triangles (4 and 5)?
  • What are the units or multiples used?

If possible, please give an example of what one of these is indicating. I mean like: so many feet of some kind of altitude.

  • $\begingroup$ In each of the examples in that answer, the outer (small) hand indicates thousands of feet and the inner (large) hand indicates hundreds of feet. Of course, some altimeters indicate metres. the first example shows 2780 feet, the second 9120 and the third 160. $\endgroup$
    – Simon
    May 21, 2017 at 6:48
  • 1
    $\begingroup$ Sorry for the confusion. I wasn't clear enough about the pointers I was referring to. In the image Federico posted below, I would like to know what the triangles are where the outside one is between the 8 and 9 and the inside one is just to the left of the 0. $\endgroup$
    – curious_1
    May 21, 2017 at 8:38

2 Answers 2


I found some sources of definite information, so I will answer this to confirm the suggestion made in a comment that Federico referred to in his answer. Also to fill in the details of what the OP asks.

Triangles 4 and 5 indicate the altitude difference between ISA pressure at sea level and the pressure set in the Kollsman window.

TM 1-205 Air Navigation 1940, Chapter 1, Section VII, Paragraph 52, Altimeter, beginning on page 66, at https://archive.org/details/TM1-205 has an explanation about these triangles, or as they were called "reference markers".

This post at https://www.pilotsofamerica.com/community/threads/blue-and-yellow-does-anyone-remember.79471/ beginning at post #11 discusses altimeters with these markers. Post #25 has a link to an article in the January 1948 issue of Flying Magazine that has a very good explanation of altimeters and these reference markers.

Triangle 5 shows if an altitude difference is higher or lower by which side of the 0 it is on. A higher difference is to the right of the 0 and a lower difference is to the left of the 0. This is in thousands of feet.

Triangle 4 reads in hundreds of feet. Which way you read it depends on which side of the 0 triangle 5 is on.

In the image in this thread, the altimeter setting is 30.1. To see what the difference should be, I used TM 55-6610-247-40 Table 3-1. Altitude Pressure Table, Feet VS Inches of Mercury. I got this TM from http://www.liberatedmanuals.com it says that the altitude difference would be -165 feet.

Triangle 5 is to the left of the 0, between it and -200 feet on the thousands scale. Since triangle 5 is on the left side of the 0, triangle 4 is read from that side and shows -160 feet.

A couple more examples from the thread referenced in the OP What type of altimeter is this?

In the top image the Kollsman window is blanked out. According to TM 1-205 page 68, (3), this is because the limit of the pressure range has been reached. In this case, at the upper limit. The triangles indicate approximately -2380 feet, which would be in excess of 32 inHg.

The bottom altimeter is set to 29.16. According to the table mentioned above, the difference is 711 feet. The triangles are showing this value. Triangle 4 is read from the right side of the zero, because triangle 5 is to the right of the 0.


Taking the image straight from the answer to the question you linked:

enter image description here

What do these pointers indicate?

What units do they represent?

It is written in the image: the outer is hundreds of ft, the intermediate is thousands and the inner is tens of thousands.

For example, the altimeter in this image shows an altitude of 9120 ft.

If, on the other hand, you mean the little triangle that is below the "0" in this image, from @mins comment,

it likely indicates the difference between ISA pressure at sea level and pressure set in window, expressed in altitude. It would turn with the setting disc.

  • 2
    $\begingroup$ What about the little notch on top of the tick mark at 8.4, marked with a "4" in the question? $\endgroup$ May 22, 2017 at 14:29

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