Available translations: English
Integrated Hydrological Digital Terrain Model

This is a 50m grid interval digital terrain model (DTM) derived from 1:50,000 scale mapping from Ordnance Survey and Land & Property Services (previously OSNI). The IHDTM is comprised of five separate gridded datasets which can be licensed together or separately.

The five gridded datasets are:

Elevation (height), Outflow and Inflow drainage directions, Cumulative Catchment Area and Surface Type.

Each IHDTM grid is hydrologically consistent with the other IHDTM grids and with the CEH 1:50,000 Watercourse Network dataset.

IHDTM Elevation

This is a hydrologically consistent DTM providing height in units of 10cms. Extensive work has been done to ensure that the hydrological behaviour that will be predicted by running models on this DTM correspond as far as possible with reality.

IHDTM Outflow drainage direction

The Outflow grid was developed from the Elevation grid but with substantial modification to ensure hydrological consistency. The Outflow grid has been modified from the raw output of flow direction algorithms to enforce the "correct" direction of flow based on other sources of knowledge aside from the adjacent eight cells in the Elevation grid itself. For example, where there is no change of elevation between cells it might not be possible to determine the correct flow direction using automated algorithms. This has been an iterative process and makes the IHDTM grids unique.

Flow direction is provided by allocating a value to each cell that represents one of the surrounding eight cells into which water from that cell will flow.

IHDTM Inflow drainage direction

The Inflow grid has been obtained directly from the Outflow data and is used in order to accelerate some applications, in particular catchment boundary definition.

IHDTM Cumulative Catchment Area

Cell values in the Cumulative Catchment Area grid represent the total number of cells that drain into that cell. This grid was derived from the Outflow grid.

IHDTM Surface Type

Cell values in Surface Type grid represent the most hydrologically significant surface type existing in the 50m square centred on the grid point, where the types rated in increasing order of significance are: land, river, lake, intertidal, sea. -1 undefined.

Technical features

  • Scale: 50m gridded dataset
  • Supply Format: Arc/ Info Grid ASCII
  • Coverage: UK
  • Elevation Units: 100mm



Morris DG and Flavin RW, 1990. A digital terrain model for hydrology. Proc 4th International Symposium on Spatial Data Handling. Vol 1 Jul 23-27, Zürich, pp 250-262.

Morris DG and Flavin RW, 1994. Sub-set of UK 50m by 50m hydrological digital terrain model grids. NERC, Institute of Hydrology, Wallingford.

Obtaining a licence

To obtain a licence, please contact the UKCEH Data Licensing Team by email or filling in our data request form:

Email: spatialdata@ceh.ac.uk

Web form

Please fill in our form to request map data

Academic research and education

IHDTM data are available for free to users of the EDINA Environment Digimap service, which includes staff and students at more than a hundred UK further and higher education establishments.