The relevance of Project Zero, Sea Level / Altitude / AHD, Reference Levels & Home Story Settings
Looking at how to get a building to relate to sea level in ARCHICAD is an interesting topic for discussion and in my opinion has been prone to interpretation. I have had many a debate with other ARCHICAD users over these past few years regarding the “correct” way to use it. My take on the subject is that I use it the way GRAPHISOFT intended it to be used. A big game changer was one of the new features of ARCHICAD 16.
Better Project Location/Project North Definition > Define a Project Altitude relative to Project North; optionally use the Altitude value as a fixed Reference Level in Project Preferences, available in all dialogues that refer to a Reference Level
The relevance of this is that the field “Altitude” found in Project Preferences > Project Location sets the “Elevation” field for “Sea Level” found in Project Preferences > Reference Levels. In other words, they are directly correlated.
The most important part to note is that the actual elevation values of any model elements are always calculated from Project Zero and that reference levels are for display and input aids. Another way to put it is Project Zero never moves up or down; it is a fixed reference point. Changing the value Altitude (Sea Level) does not affect Project Zero. In other words, the building related to Project Zero does not move, but Sea Level does.
Let us look at the term or definition of “Project Zero” before continuing.
When teaching ARCHICAD to new users, I like to explain that Project Zero = Reduced Level in surveying terms.
In other words, Project Zero is the adopted datum plane for the start of the building model; it is horizontal and commonly assigned to the top elevation of the Ground Floor slab, placed on Story No. 0 – Ground Floor (street level).
For those American readers of this blog, I would like to point out that the localized version of ARCHICAD USA; Story settings start with story No. 1 – 1st Floor. It is of course due to the difference in British and American English: the naming of floors in buildings.
In British English, the floor of a building at street level is called the ground floor. The floor above it is the first floor, and the floor below is known as the basement.
In American English, however, the floor at street level is usually called the first floor. Go up one floor and you are on the second floor (which, of course, is the first floor for the British). The floor below street level is called the basement, the same as in British English.
In other words for America, Project Zero is assigned to story No. 1 – 1st Floor (street level).
Just to reiterate the use of “Altitude” has been with us since it was introduced in ARCHICAD 16. Which was announced on May 2, 2012, more than 4 years ago
A functionality improvement for ARCHICAD 19, was the inclusion of “Altitude (Sea Level).” My thanks go out to James Badcock, who is a Product Designer at GRAPHISOFT HQ, who advised me that he was responsible for putting in the request to the Product Team to insert the words “Sea Level” after Altitude.
Note: You cannot rename the Project Zero or Altitude reference levels.
Note: Reference Levels are display and input aids only. Changing their value has no effect on placed elements, whose actual elevation is always calculated from Project Zero. The exception is Grid lines in Section view, whose position changes with the Reference Level.
But you can rename the 2 provided Reference Levels and change their Elevation value. This is especially useful when working on a building that has split levels.
Reference levels can also be used as reference levels for showing elevation (Z) values in the Tracker. When entering a distance using Tracker, keep pushing Tab to get to Z Coordinate and click on the small arrow to show “Elevation Value Origin”.
What does this mean for the user, and how did GRAPHISOFT envisage that it should be used? The answer lies in the simple fact that when looking at Story Settings, there is no need to insert any value into Story Settings to get the model to relate to Sea Level.
Looking at it from an Australian perspective and the use of AHD or Australian Height Datum. According to:
The Australian Height Datum is a geodetic datum for altitude measurement in Australia. According to Geoscience Australia, “In 1971 the mean sea level for 1966-1968 was assigned the value of 0.000m on the Australian Height Datum at thirty tide gauges around the coast of the Australian continent. The resulting datum surface, with minor modifications in two metropolitan areas, has been termed the Australian Height Datum (AHD) and was adopted by the National Mapping Council as the datum to which all vertical control for mapping (and other surveying functions) is to be referred.
It is my opinion that “Australian Height Datum” or AHD for short is just another way to say “sea level”. And that in ARCHICAD, the use of the value field for “Altitude (Sea Level) is the same as AHD as it was assigned the value of 0.000m.
The use of “Story Settings” in ARCHICAD INT is to represent the logical story structure of the building starting with story 0 = Ground Floor = Project Zero. Then Story 1 = First Floor, Story 2 = Second Floor etc. Therefore when looking at Story Level Lines, they follow the same logical progression. Inserting a story below Story 0, results in Story -1 which can be used for Basement / footings / foundation.
Looking back, to before “Altitude” was introduced in ARCHICAD 16, namely versions 15, 14, 13, etc. there was still one of the two Reference levels (1st or 2nd) that could be substituted and renamed to AHD or Sea Level.