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OS MasterMap Imagery Layer frequently asked questions
What is the difference between geocorrected and orthorectified imagery?
OS MasterMap® Imagery Layer is fully orthorectified to provide a high level 1.1 m RMSE accuracy and correct alignment with our other detailed data.
Geocorrected imagery has been fitted to mapped features and does not include a digital terrain model (DTM) to compensate for ground height variations within the image. It has the advantage of being inexpensive to produce, and gives a good approximate fit to vector map data. But since it is not controlled to the National Grid or adjusted to compensate for the ground height variations, it does not inherently provide positionally or geometrically accurate imagery. Update is also a problem since the controls, being based upon mapped features rather than true location, are not necessarily stable.
Orthorectified imagery is created using National Grid control points and a DTM. This means that the distortions inherent in a flat photograph of a three-dimensional object (the earth's surface) are more completely and accurately corrected. An orthorectified image is positionally more accurate, and geometric fidelity (shape) is retained in all terrain, including hilly areas. Also, a consistent scale is maintained throughout the image (as per a map), and future imagery updates will be in the same position.
How closely does Imagery Layer align with other OS MasterMap layers?
All of our orthorectified imagery in OS MasterMap Imagery Layer uses direct National Grid control or GPS with a suitable transformation in order to achieve our specified positional accuracy.
This means that features will align very closely between the image and vector data, within the expected tolerances. The imagery is also revised at the same time as OS MasterMap Topography Layer as part of our Integrated Revision Programme to ensure that the data layers are synchronised. Together these processes provide imagery that is positionally correct and temporally consistent to maximise interoperability between OS MasterMap layers and your own data.
What is the level of quality that Ordnance Survey has applied to its imagery?
Ordnance Survey has invested considerable resources and worked closely with its imagery suppliers to provide orthorectified imagery that achieves the consistently high quality demanded 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 that 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.
Go to FAQ page on level of quality of the imagery
How can I find out the currency of my area of interest?
As part of our ongoing commitment to maintenance we continuously refresh OS MasterMap Imagery Layer as part of our cyclic integrated revision schedule.
How frequently will OS MasterMap Imagery Layer be updated?
The Imagery layer provides full coverage of Great Britain, England, Scotland and Wales. We continuously refresh this on a cyclic schedule based on levels of expected change utilising our knowledge of change intelligence. Imagery is captured as part of our integrated revision programme which updates imagery alongside our detailed data, notably OS MasterMap Topography Layer. This ensures that the layers are synchronised when used together.
Go to FAQ page on update frequency
What does update cycle mean?
We aim to ensure that all imagery in the OS MasterMap Imagery Layer is scheduled for update within a rolling programme, derived from our knowledge of change intelligence of Great Britain. This does mean that some remote areas that see less frequent change on the ground might be updated towards the end of the cycle and, on average, be older than areas such as cities where change is more rapid. Our currency is market leading, 95% of the layer is five years old or less (figures from June 2010).
What format is OS MasterMap Imagery Layer delivered in?
The OS MasterMap Imagery Layer is supplied in 1 km by 1 km tiles in your choice of the following formats:
• Tagged Image File Format (TIFF);
• Joint Photographic Experts Group (JPEG);
• Enhance Compression Wavelet (ECW); and
• Multi-resolution Seamless Image Database (MrSID).
How do I georeference OS MasterMap Imagery Layer tiles?
All of the OS MasterMap Imagery Layer formats can be registered in a GIS or image-processing software. This is achieved in different ways depending on the format, either through the accompanying georeferencing files, header files or with metadata delivered with the image file.
The imagery user guide provides full technical details. Your software supplier will also be able to advise you on this matter.
In Imagery Layer, what do you mean by 'seamless imagery'?
The imagery in OS MasterMap Imagery Layer is seamless in that it is colour balanced and edgematched. This ensures that analysis of information across image boundaries is not seriously hampered by misalignment or inconsistency.
Go to FAQ page on seamless imagery
What will be the oldest imagery in OS MasterMap Imagery Layer?
No OS MasterMap imagery from the 2003 launch was older than 1999. Since then we've worked hard to gain market leading currency over full coverage of Great Britain. As of June 2010 only 5% of the layer is older than five years, with 50% less than three years old. Every month we publish our latest currency on our coverage page so you can keep track of our progress. For a full breakdown of the currency in your area of interest please contact us.
Can I obtain archived OS MasterMap Imagery Layer?
No. Ordnance Survey does not currently offer archived OS MasterMap Imagery Layer data, but an archive policy is being developed.
Will OS MasterMap Imagery Layer tiles include metadata information?
Yes, each OS MasterMap Imagery Layer tile is accompanied by a separate XML metadata file in a consistent format across every image in the layer. The metadata is extremely detailed and includes information ranging from imagery capture date to photographic lens focal length. The imagery user guide contains detailed technical information.
Does OS MasterMap Imagery Layer contain TOIDs?
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.