Elevation and Depth data

Also see Walton on the Naze tides – variation

LIDAR is a Laser-based surveying technique that provides elevation data across a land area. There are various types of survey data that can be downloaded and then processed to show a given area. The raw data will include buildings and vehicles, this is the DSM version. Processing is done to remove these and just give the terrain, this is the DTM version. The height is expressed in metres above the Ordnance Datum (AOD), and depressions are negative. This is essentially the Mean Sea Level (MSL) for the area. The precision of the data varies, large areas tend to be in large horizontal squares and small areas in more precise squares or tiles, these can be as small as 0.5m square.

Bathymetry data is available from HHA and other sources. This is expressed in metres below Chart Datum (negative) with drying heights positive.

Data Sources

• DEFRA Portal with LIDAR and some Bathymetry. 
• Also EU 25m DEM – low resolution but good for wide areas from Copernicus Portal.
• Best for bathymetry Admiralty UKHO
• UKDMOS is the United Kingdom Directory of Marine Observing Systems.
• Portal Medin Portal
•  Also Copernicus marine data 
• EMODnet – not so good as above. 
• seascapes_southwold-clacton_FINAL_REPORT   
• National Tidal and Sea Level Facility (ntslf.org)
• British Oceanographic Data Centre (BODC) — oceanographic and marine data

Worth a read on CRS

Detail on surveys and precision. Much local info on sea level, tide, temperature etc. in the Coastal Monitoring reports. See this one.

Excellent UKHO Guide on Bathymetry.

Note: Add CSV file to QGIS as delimited text. Set Lat, Long, and Depth fields. Then use the contour plugin to generate contours.  

DEM of Seabed

The only DEM found for offshore is here. OpenDEM which leads to GEBCO data download

OD vs CD

Because the range of tidal height around MSL varies, generally larger going south, the relationship between Ordnance Datum and Chart Datum is variable with the correction ranging from Sheerness -2.9m to Lowestoft -1.5m. This needs to be added algebraically to Height OD to get to CD. For local -2m is a reasonable approximation.

For this and other reasons, it is not straightforward to combine Bathymetry and LIDAR data along the coast. Fortunately, there is a dataset called Surfzone, which merges the land and sea-based data for most of the coastline that is of interest. To use tidal data with this it is necessary to adjust for the difference in OD and CD at that location.

MSL is equivalent to OD Newlyn (it might have been Felixstowe, but Newlyn got the job)¹MSL history – see Maritime ArchaeologyTrust

Felixstowe Ordnance Datum is Newlyn -1.95m, Lowestoft is Newlyn -1.5m²NTSLF Chart datum & ordnance datum | National Tidal and Sea Level Facility (ntslf.org)

If for example, you have a tidal height for Felixstowe of 10m above CD and want to convert it to be relative to OD, then 1.95m tidal height (i.e. MSL) – 1.95 = 0m AOD or 10m + ( -1.95m ) = 8.05m

Chart Datum of Harwich Harbour, a level of 2.02 metres (6.62 feet) below Ordnance Datum (Newlyn), is approximately 0.1m above the level of the lowest astronomical tide.

Chart Datum of Walton on Naze a level of 2.16 metres ( 7.09 feet) below Ordnance Datum Newlyn.

Chart Datum of Woodbridge should be about half of Spring Range=1.8m below Ordnance Datum Newlyn.

From Suffolk SMP2 Sub-cell 3c Policy Development Zone 6 – Orford Ness to Cobbold’s Point.

Harwich Area Tidal Range

Sea level at Woodbridge – GOV.UK (check-for-flooding.service.gov.uk)

Historical W-o-N from PSMSL Data and Station Information for WALTON ON THE NAZE (psmsl.org)

Here are surge forecasts for Felixstowe for 48 hours ahead.

Colour Style for Diagrams

Generally, the interest is in the sea depth and level relative to the land. To account for the change in range then the colour set is shifted every few miles to maintain the meaning. For shallow draught vessels 2m is considered just navigable.

CD	OD	Comment	Colour
-20			Blue 6
-15			Blue 5
-5			Blue 4
-2		Always navigable	Blue 3
0	CD+Var	MSL	Blue 2
2		Navigable HWS	Blue 1
		MHWS Shoreline	Blue 0
		Surge line = MWHS+1m	Violet

• 1
  MSL history – see Maritime ArchaeologyTrust
• 2
  NTSLF Chart datum & ordnance datum | National Tidal and Sea Level Facility (ntslf.org)