Ordnance Survey has invested considerable resources and worked closely with its imagery suppliers to provide orthorectified imagery that achieves the consistently high quality expected by our customers.

Unlike other data sources, such as topographic data, imagery can be very subjective. It is difficult to quantify specific tolerances for atmospheric conditions, such as haze. We ensure our suppliers adhere to a detailed set of quality levels for the key aspects of imagery with an emphasis on data quality over image appearance alone. For instance priority is given to ensuring detail such as road markings can be seen within shadows and to positional accuracy.

These acceptable quality levels are summarised in the user guide and include:

• the photography and how it is captured;
• the controls and height models used for orthorectification; and
• geometric fidelity (shape), absolute accuracy (position), radiometric accuracy (colour) and image appearance (contrast, haze, cloud cover and distortions).

More than 97% of our imagery is less than five years old; 90% is less than three years old. Check imagery currency on our interactive map.

As the OS MasterMap Imagery Layer is a raster product, unlike the other (vector) layers in the OS MasterMap product range, TOIDs® are not part of the data specification. However, OS MasterMap layers are designed to be used together in order to create a synchronised tool for analysis of many different types of data in a GIS. Thus other layers can be drawn over imagery to provide information, such as TOIDs.

However, within these images there may be small patches of 'traditional' ortho (with building lean) which have been introduced to resolve geometric or image appearance/artefact issues. Whether 'True' or 'traditional' Ortho, buildings should always be geometrically correct, located in their correct geographical position and suitable for analytical interpretation.

However, on occasion and under certain circumstances, these rules are relaxed. For example, if opportunities to capture new imagery have been limited by poor weather and existing imagery in the Imagery Layer is deemed out-of-date, then imagery will be accepted provided detail is visible on the ground.In these circumstances, image currency takes precedence over image appearance.