Vertical and Horizontal Datum

To place your data correctly on the globe, AO needs to know its horizontal datum (where it is) and its vertical datum (what its heights or depths are measured from). This article explains both, and how to set them when you visualize a file.

What they are

A horizontal datum defines latitude and longitude, so AO can put your data in the right place. A vertical datum is the surface that heights and depths are measured from, such as the WGS 84 ellipsoid, mean sea level or mean lower low water. The same seabed has a different depth value against each one, much as a temperature reads differently in Fahrenheit and Celsius. You can convert between them, but you need to know which one you started with.

NOAA has more background on datums and vertical datums.

How AO uses them

AO shows all data together on one 3D globe, so it displays everything against WGS 84, the global standard. When you visualize a file, AO transforms a copy of it to WGS 84 for display. Your original file isn't changed. How accurate the transformation is depends on the source datum and the location.

AO doesn't apply sound velocity (SVP) or tide corrections to your data. Apply them in your processing software before you upload.

If the vertical datum is wrong, a grid appears above or below where it should be, often with a cliff between it and the surrounding base terrain.

To show depths relative to mean sea level instead of WGS 84, click your initial in the top right corner, choose My Account, open Settings, and set Elevation Datum to EGM2008 Geoid (Mean Sea Level).

Setting the datums when you visualize

AO uses the Coordinate Reference System (CRS) setting. It looks for the CRS in this order:

The file. Many formats store the horizontal CRS in the file header; AO shows it as the default.
The project. If the file doesn't say, AO uses the project's default CRS. We recommend setting one for each project, and noting the datums in the project description.
You. If neither has it, AO asks you to choose one.

To change the CRS, choose the pencil icon beside it and search by name or EPSG code. Pick the entry with the right units (meters or feet) and direction (heights up or depths down): many coordinate systems have one entry for each. AO checks that a transformation exists for your choice and location before it visualizes, and tells you straight away if it doesn't.

Vertical datum

Files rarely store their vertical datum. It's usually in the survey's documentation, or the surveyor can tell you. AO can transform from:

Tidal datums: mean lower low water (MLLW), mean low water (MLW), mean high water (MHW) and mean higher high water (MHHW), using NOAA VDatum, where VDatum covers the area.
Mean sea level: search for "MSL" and choose MSL height for heights or MSL depth for depths.
The International Great Lakes Datum 1985 (IGLD 85).
Other geoid-based and ellipsoidal datums, including WGS 84.

Five depth datums can't be used for elevations: LAT depth (Lowest Astronomical Tide, a common chart datum outside the US), LLWLT depth, ISLW depth (Indian Spring Low Water), MLLWS depth and MLWS depth. AO says "This vertical datum is not supported for elevation ingest. Choose a tidal or orthometric datum." For data on one of these, use a Custom Vertical Offset (below). If you don't know the vertical datum, MSL is usually a reasonable choice.

Custom vertical offset

If none of these fits, use the switch beside the CRS to enter a Custom Vertical Offset instead: a shift applied to the whole dataset to line it up with AO's terrain. It may take some trial and error. With an offset, also set Vertical Units and Z Convention to match the file: they start at your display units and Positive Up (Vertical Units stays hidden if you chose a vertical CRS before switching to the offset).

Without internet access

The CRS picker works offline, for example on AO OnBoard, and AO still checks transforms on the AO server. If that check doesn't answer within a couple of seconds, or fails, AO shows Unable to verify transform offline. While it shows, Generate Visualization is unavailable. See Using AO offline.