Integrated Hydrological Digital Terrain Model (CEH)

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.

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.

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.

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.

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.