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Type Identifier (TID) Grid

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The GEBCO bathymetric grid is accompanied by a Type Identifier (TID) Grid, the function of which is to identifier the source data type that the corresponding grid cell in the bathymetric grid is based on.

The aim is to allow users to assess the ‘quality’ of the grid in a particular area, i.e. if it is based on multibeam data, singlebeam data or on interpolation, etc.

TID grid coverage for the GEBCO_2019 Grid, colour-coded for TID value

The above image shows the TID grid coverage, colour-coded for TID value for the GEBCO_2019 Grid.

How the TID Grid can be used

The following images show examples of how the TID Grid can be used to highlight the types of data that areas of the GEBCO grid are based on.

Why do some areas of the GEBCO grid look more detailed than others?

Using the GEBCO Type Identifier (TID) Grid

Figure 2A shows a section of the GEBCO grid. The region in the upper left of the image is based on a database of soundings with interpolation between them guided by satellite-derived gravity data (the base grid), while in the centre and lower right it is based on multibeam data.This difference in data source is reflected in the level of detail shown in the bathymetric grid. Figure 2B shows the TID grid for this area and the coverage of the various types of data used to generate the GEBCO grid in this region. This helps to highlight the areas that are based on measured data and those that are based on interpolation.

Defining areas based on different data types within pre-generated grids

Coverage of ship tracklines (black) overlain on the GEBCO_2019 Grid in part of the Pacific Ocean. These are the lines along which depth measurements have been taken. Between the tracklines, the GEBCO_2019 Grid is based on interpolation, largely guided by satellite-derived gravity data. This highlights the need for GEBCO to continually update its bathymetric grid with new contributions of data to help fill the data gaps.

In Figure 3A the area shown in pink is a region of the GEBCO Grid based on a pre-generated grid, i.e. a grid that has been generated from multiple data sources. It is not possible to distinguish between the different source data types and the areas based on interpolation. It may appear that the whole region is mapped.

In Figure 2B, the individual source data sets are now coded for their data type. This makes it possible to identify regions based on multibeam and single beam data and those based on interpolation.

Find out more about the TID grid and how it can be generated from Chapter 17 of the GEBCO Cook Book.

TID codes

The table below details the coding of the GEBCO Type Identifier (TID) grid.

TID Definition
0 Land
Direct measurements
10 Singlebeam - depth value collected by a single beam echo-sounder
11 Multibeam - depth value collected by a multibeam echo-sounder
12 Seismic - depth value collected by seismic methods
13 Isolated sounding - depth value that is not part of a regular survey or trackline
14 ENC sounding - depth value extracted from an Electronic Navigation Chart (ENC)
15 Lidar - depth derived from a bathymetric lidar sensor
16 Depth measured by optical light sensor
17 Combination of direct measurement methods
Indirect measurements
40 Predicted based on satellite-derived gravity data - depth value is an interpolated value guided by satellite-derived gravity data
41 Interpolated based on a computer algorithm - depth value is an interpolated value based on a computer algorithm (e.g. Generic Mapping Tools)
42 Digital bathymetric contours from charts - depth value taken from a bathymetric contour data set
43 Digital bathymetric contours from ENCs - depth value taken from bathymetric contours from an Electronic Navigation Chart (ENC)
44 Bathymetric sounding - depth value at this location is constrained by bathymetric sounding(s) within a gridded data set where interpolation between sounding points is guided by satellite-derived gravity data
45 Predicted based on helicopter/flight-derived gravity data
46 Depth estimated by calculating the draft of a grounded iceberg using satellite-derived freeboard measurement.
70 Pre-generated grid - depth value is taken from a pre-generated grid that is based on mixed source data types, e.g. single beam, multibeam, interpolation etc.
71 Unknown source - depth value from an unknown source
72 Steering points - depth value used to constrain the grid in areas of poor data coverage