Last modified 11 months ago Last modified on 2019-04-08 10:49:51

Intro Datums and how we use them

The need for surveying in targets for Sturcture from Motion (SfM) analysis starting with Sandwich in 2015. None of our previous data paid any particular attention to datum used, particularly for elevations/depths. Consequently, for most of our older data, 3-d accuracy is poor. Attributes like NAVD88_elevation_ref and position_datum won't be present in files that didn't involve some form of surveyed positions. Data files from Sandwich and West Falmouth newer than 2015 will have these attributes, since the sites were surveyed.

We use WGS84 for horizontal, and NAVD88 (2011) for vertical datums.

In newer files with surveyed locations, the NAVD88_elevation_ref global attribute contains the surveyed elevation. This may be to an OPUS site or a place on or near the sensor. In most cases this will not be the actual elevation where the observation was made. In the case of Sandwich, with a dwave mounted on a disk deployed at the groin, the mooring log should say where the measurement was made. In the case on 1093, it was made to the top of the case, and the elevation was -1.94. To this measurement, you need to subtract half the case diameter to get to the measurement_elevation that goes in the "depth" variable. (our code uses WATER_DEPTH-initial_instrument_height to get "depth" if NAVD88_elevation_ref is NOT present and NAVD88_elevation_ref-sensor_offset_ref if it is.

When a surveyed position and elevation are available, WATER_DEPTH and initial_instrument_height are retained in the file, but have less accuracy than the surveyed attributes. We convert the NAVD88 water_depth to MLLW using NOAA's vdatum tool, and insert the result into WATER_DEPTH the global_atts file prior to processing. The global atts for West Falmouth 2017- 10942 looks like this:

WATER_DEPTH_note: "(meters MLLW, based on surveyed dock elevation of 1.899 m NAVD88 - 3.18 m offset to seafoor)"
NAVD88_elevation_ref: 1.899