- 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. 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.
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. These acceptable quality levels are summarised in the user guide and include:
- the photography and how it is captured;
- the standard of equipment used for scanning the imagery;
- the controls and height models used for orthorectification; and
- geometric fidelity (shape), absolute accuracy (position), radiometric accuracy (colour), image appearance (blemishes and obstructions) and mosaicking.
All suppliers are required to pass a detailed accreditation programme to ensure that they can consistently deliver to specification. As new imagery is flown it is rigorously tested against the acceptable quality levels to maintain standards throughout the production flowlines. Any imagery that does not meet the specification is rejected.
- What takes precedence, image appearance or image currency?
Great care is taken to ensure that the images in the Layer meet our detailed Image Appearance criteria. 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.
- How much of the Imagery Layer is 'True' Ortho?
The Imagery Layer is a mixture of 'true' (without building lean) and 'traditional' (with building lean) orthorectified images. All imagery since 2015 are 'true' orthorectified. However, within these images there may be patches of 'traditional' ortho 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.
- 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. We publish an interactive map that shows when different parts of the country were last flown.
For more detailed information, including planned flying and progress of imagery in production, please contact your account manager.
- 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.
We typically plan to fly up to 80 000 sq km during the flying season, from March until November each year. Achieving our update targets will, of course, be dependent on weather conditions, quality checks and flying restrictions. Thus we are unable to guarantee imagery captured until it has been successfully processed.
- What formats are available for OS MasterMap Imagery?
The OS MasterMap® Imagery Layer is supplied in 1km x 1km tiles in your choice of the following formats:
- Tagged Image File Format (TIFF);
- Joint Photographic Experts Group (JPEG); and
- Enhance Compression Wavelet (ECW).
All files are accompanied by the relevant georeferencing files to enable integration in a GIS. The main features are summarised below.
TIFF is one of the most commonly used lossless image formats. TIFF is primarily designed for raster data interchange, and is supported by numerous image-processing applications.
The other two formats are compressed to reduce their file size and facilitate data management. These formats are recommended for very large or national coverage orders. As file compression can vary, the following file sizes are average.
A JPEG image uses a lossy compression format. It is designed for compressing either full-colour or greyscale images of natural, real-world scenes and so works well on photographs, becoming the standard format for web site applications.
The ECW raster image uses wavelet compression software which has been designed to compress, display, Internet serve and distribute imagery. The key characteristic of such compressions is that they are able to preserve a very good quality of the image with high compression ratios.
Please note, only one format can be supplied per order. For ECW files, a plug-in is normally required. Customers are advised to consult the suppliers of their image viewing software to discuss format requirements prior to placing an order for imagery.
Further technical details are available in our user guide.
- What processes are undertaken to ensure a consistent image?
Each individually flown block is colour balanced and adjoining blocks are edgematched to ensure that analysis of information across image boundaries is not seriously hampered by misalignment or inconsistency.
Variations because of time of day differences (for example, lengthening shadows, colour changes), time of year differences (for example, vegetation changes, river levels) and age differences (for example, new developments, and road changes) mean that some variation is inevitable and an overzealous attempt at imposing an artificial evenness would be misleading.
Great care is taken to ensure that imagery is 'stitched' together carefully to avoid distortions in features such as buildings. We collect imagery in strips with a generous overlap between each photograph and then use seamlines to select the centre of each image.
- 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. A national set of World files is available opposite.
The imagery user guide provides full technical details. Your software supplier will also be able to advise you on this matter.
- Can I obtain archived OS MasterMap Imagery Layer?
No. Ordnance Survey does not currently offer archived OS MasterMap Imagery Layer data.
- 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.
- How old is imagery in the OS MasterMap Imagery Layer?
In line with our release schedule, every quarter 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.
The Imagery Layer is digital aerial photography of the highest quality that is expertly colour-balanced and corrected for geometric distortions (orthorectified).
*Please note: A small proportion of 2016 imagery in and around London has been accepted into the Imagery Layer that does not strictly meet the specification. These areas have been created as traditional not true orthorectified images. This means that there is some building lean in the images. In addition, in some areas, the image quality is below that which we normally look to achieve, however you can still see ground detail.