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I've discovered, that I have no idea, how height is usually represented in GIS.

For example, let's say we have software, that lets you create a mission plan for an UAV such as

  • take off
  • go to point x1
  • go to point x2
  • ...
  • land

Where "point" is represented by

  • latitude
  • longitude
  • height

Latitude and longitude are "intuitive" (there's no question about what they actually mean, once you select a geodetic datum). However, height is not. How can one represent it? Things, that come to mind:

  • height in relation to takeoff point
  • height in relation to "sea level"
  • height in relation to some other reference point
  • distance from the center of the Earth (I mean, why not?)

Is there some canonical way of representing heights, preferrably, a way that is not tied to the problem domain (UAVs, etc)?

Of course, I am ready to accept an answer, that will point me to some reference / literature on the topic, instead of answering the question directly. I am, in fact, quite ignorant about GIS, so I don't have much of an idea even which keywords to search for.

closed as too broad by mgri, nmtoken, Andre Silva, Fezter Mar 9 '17 at 11:40

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Latitude and longitude are "intuitive" (there's no question about what they actually mean, once you select a geodetic datum) I'm not sure that's entirely correct, but nonetheless just as there are 2D coordinate reference systems, so there are 3D coordinate reference systems, and compound coordinate reference systems that include a height/depth component. – nmtoken Mar 9 '17 at 9:27
  • I don't see how "Is there some canonical way of representing heights?" is "too broad". I mean, it is even possible to answer it in a binary way as yes / no. How can it be more specific? – shylent Mar 9 '17 at 12:25
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Height could be represented as any of your suggestions. A raster is just a value grid after all and what it represents should be explained in some metadata. Conventionally, however, it is common to represent heights in a digital elevation model (DEM) as 'bald Earth' height above sea level (ie no trees or buildings for instance). This would be the case in SRTM data for instance. Be careful of digital surface models though (DSM) where the heights are not 'bald Earth'. This would be the data you commonly get from LIDAR for instance.

The best advice is to read the metadata for the dataset you intend to use. Also don't rely on terms like DTM, DEM etc as different people use them in different ways and often interchangeably. Some will insist that a DTM is a DSM and others that it is a DEM (as the terms are used here) for instance (and much argument will ensue) - so really, the metadata is your friend!

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The term you are looking for is 'Vertical Datum'.

It is the same really as co-ordinates such as latitude & longitude. These X & Y measurements (be it degrees or metres) only make sense when you know the starting point. It is exactly the same for the height, or Z value. As long as you know the level at which you are starting, all Z values have a level of reference to be measured from.

With marine survey, vertical datums are based on sea level (for example, lowest astronomical tide is commonly used for Chart Datum, or the more complex VORF). On land, it is common to use mean sea level along with a geoid model (e.g. Ordnance Datum Newlyn), or one of the ellipsoid models of the earth.

For more info, see wikipedia.

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Other answers addressed the way to represent elevation. What I'm doing with my answer is try to address the height reference issue.

When planning an aerial survey mission, we generally refer to height above ground, whether you're flying a manned airplane or a UAV. This is critical to the mission on hand as it is the height above ground that will determine the photo scale and consequently your GSD (Ground Sample Distance) or resolution, or pixel size. This is even more critical for UAV surveys for safety reasons, as these platforms fly at a relatively low altitude, (you don't want your vehicle to hit a nearby hill) and for the photo scale (a ground height variation of 50m when you're flying at 200m above ground has more effect than the same variation when you're flying at 3,000m).

Now when planning a flight, you want your vehicle to fly each line at a fixed altitude above sea level, so in case there is some ground variation, you will have to keep that in mind regarding the effect on scale.

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