How To Make An Orthophoto

Transcription

1 ISSUE 2 SEPTEMBER 2014 TSA Endorsed by: CLIENT GUIDE TO DIGITAL ORTHO- PHOTOGRAPHY The Survey Association s Client Guides are primarily aimed at other professionals such as engineers, architects, planners and clients in general. They are not intended to go in depth into practical issues but to act as a basic guide on a particular topic and in particular, on procedures and regulations which may govern how a particular aspect of the survey is carried out. 1 Introduction Digital Orthophotography is the method of combining the image characteristics of an aerial photograph with the geometric quality of maps. Traditionally orthophotos were captured with an analogue camera mounted in an aircraft or helicopter, and then were scanned to convert them into digital images which could be manipulated to remove the effects of terrain relief and scale. Now the process is simpler in that the imagery is now captured with digital cameras removing the scanning process from the production flowline. Orthophotos can also be generated from mobile capture systems and satellite images, in the same way.

2 2 What is an Orthophoto? An orthophoto, as displayed in figure 1 is a single or series of aerial photographic images in which displacements caused by scale, terrain relief and camera orientation have been removed. Orthophotos have the high visual information content and familiarity of a photograph but contains the geometric qualities of a map and can be used in a multitude of GIS and mapping applications. Orthophotos can be presented as hard-copy or as digital image data. The hard-copy is presented as a photograph and can have contours, grid information or other map details overlaid. Digital orthophotography can be used for interpretation, measurement, quality assurance, or combined with vector map data as a backup to a GIS or CAD model. Currently, orthophotos are generated in a digital environment, the digital image being rectified to an orthographic projection by processing of each individual pixel through photogrammetric computations derived from photo identified ground control points, camera calibration and a digital elevation or terrain model. These functions are normally performed on a digital photogrammetric workstation. Figure 1: True Colour 25cm Resolution Orthophoto 3 Types of Digital Orthophoto? There are different types of digital orthophoto available. The end use of the imagery is a key determinant of the type of orthophoto to be created and used. Orthophotos can be generated from imagery captured from aircraft/helicopter either as analogue imagery which is scanned, or as digital images. Due to the advances in technology over the last 10 years the majority of aerial capture is nowadays undertaken with a digital camera. Orthophotos can also be generated from satellite imagery with new generation very high resolution satellites which can offer digital imagery at 30cm - 50cm resolution. Some aerial survey cameras can also capture imagery with a near infra-red channel such as the UltraCam X system which can be used to create colour infra-red orthophotos, as shown below in Figure 2. 2

3 Figure 2: Colour Infrared Orthophoto The source of orthophoto will be determined by the project requirements, and the user should consider the scale and resolution of the imagery as well as the area to be covered and also timeliness of capture. In addition to traditional vertically captured orthophotos, new technologies are now providing means to capture imagery on the move with mobile mapping systems such as the Leica Pegasus system which captures LiDAR point clouds and imagery at the same time. 4. Equipment Orthophotos are now normally captured and created by aircraft mounted aerial survey camera systems such as the Leica ADS100 or UltraCam Eagle system which provide the capability to capture full colour imagery together with a near infra-red channel, with increasing swath widths of up to pixels. Another part of the system is the Inertial Navigational System (INS) which records the position and orientation of the system while the camera is taking the images. The INS works by combining information from Global Navigation Satellite Systems (GNSS) receivers with an Inertial Movement Unit (IMU) to calculate highly accurate positional and orientation estimates for the trajectory of the system. While INS are not compulsory more and more modern systems will use them, and in certain circumstances this may be enough to position the imagery without having to rely on additional ground control. 3

4 Figure 3: Leica ADS100 Airborne Digital Sensor (left) and Vexcel UltraCam-D Digital Aerial Photography system (right) Digital cameras come in two types: Digital frame cameras; for example the Vexcel Ultracam and Intergraph DMC ranges of cameras Push broom sensors; such as the Leica ADS80, Jena JAS 150 and Wehrli Associates 3-DAS-1 All airborne digital cameras can capture imagery in a variety of formats and pixel resolutions. Cameras such as the Intergraph DMC and Vexcel UltraCam are considered large format digital cameras and are the relevant manufactures models aimed as alternatives to the traditional film survey camera. Cameras produced by manufactures such as Applanix, Leica and IGI Systems are medium format digital cameras. Both the DMC and Vexcel UltraCam cameras collect Black and White imagery (B/W), True Colour (RGB) and near Infrared Imagery (NIR) simultaneously, but the B/W is captured at a higher resolution than the spectral information. The two sets of data are then merged in a process known as pan sharpening, to blend the detail from the B/W imagery with the spectral information from the RGB and NIR, to produce RGB and NIR imagery at the same resolution as the B/W. The medium format cameras mostly acquire either RGB or CIR (Red, Green, NIR) simultaneously, all at full resolution as a single image, though some medium format cameras can acquire RGB and NIR simultaneously. Satellites can also produce imagery suitable for generation of orthophotos. There are now selections of very high resolution optical satellites in orbit such as the Airbus DS Pleiades which can capture imagery at resolutions of 50cm and the Digital Globe WV3 at resolutions of 30cm. This imagery can be processed in the same way as traditionally captured aerial imagery. 4

5 5 What is required to produce a digital orthophoto? In order to produce digital orthophotos the imagery needs to be in a digital format. If archive analogue imagery is being used this needs to be scanned from the original film negatives or film diapositives using a geometrically and radiometrically precise digital scanner. For more recent archive imagery, or new surveys, the imagery should already be in the digital environment as they would have been captured with a digital imaging system. The digital images are subject to photogrammetric orientation to remove distortions associated with the aerial camera, and scale and relief displacement of the ground control points. Calibrated camera information is used to remove radial lens distortions. The relative and absolute orientation process removes parallax between two adjacent, overlapping photographs and then mathematically scales and levels the stereo-model to the ground control points. Figure 4: Pleiades satellite imagery Airbus Defence and Space This stage uses boresighting and/or aerial triangulation (referencing known ground control points which can be clearly identified on the imagery). Boresighting is the process of removing systematic errors between the alignment of the cameras focal plane and the navigation systems, which commonly integrate with most installations of modern digital cameras. This allows the images to be directly geo-referenced using only the airborne navigation solution therefore reduces the number of ground control points required in the aerial triangulation process. At this stage it is necessary to supply a Digital Elevation Model (DEM) or Digital Terrain Model (DTM) to orthorectify every pixel for changes in relief. This is required because an array of pixels that is positioned at the top of a hill will have a different scale to an array of pixels further away from the camera in a valley. A DEM is normally associated with a regular grid of elevation data, whilst a DTM is collected using a more extensive layout where the regular elevation grid is supplemented by terrain breaklines that more accurately define significant changes in terrain data that relates to elevated features such as bridges and overpasses that require particular attention. The orthorectification process is usually undertaken in a digital photogrammetric workstation. 5

6 Figure 5: DTM and Orthophoto A typical orthophoto project will often consist of blocks of aerial photographs that have been rectified and mosaiced together. Mosaicing involves balancing the radiometric values concerning colour, brightness and contrast so that a continuous tone, seamless orthophoto is produced. The final orthophoto is often cut into manageable tiles that may correspond to National Map Agency map sheets or to an existing vector map layout. The imagery is sometimes delivered in a data pyramid, where lower resolution images of the base file are delivered together with the full resolution images. This has an advantage for the user in that the smaller lower resolution images can be used for review or coarse interpretation. The smaller files are more easily managed and viewed rapidly. The higher resolution images are still available for more detailed analysis and interpretation. 6 Accuracy versus Resolution It is essential for the users of orthophotography, particularly those with a nonphotogrammetric background to distinguish between accuracy and resolution. A product that is quoted as 25cm orthophoto may refer to either the pixel resolution of the image or the absolute spatial position to the ground coordinate system. It is unlikely that a 25cm size feature could be recognised from an image of the same resolution. Photogrammetrists generalise that the ground resolution should be half the size of the object to be resolved. If a 50cm feature needs to be identified, then the ground resolution should be no greater than 25cm. An image that is scanned at a ground resolution of 25cm may have a significantly lower accuracy if poor quality planimetric control was used to scale the product or if a low accuracy DEM was used to correct for relief. 6

7 The quality of the DEM/DTM used to orthorectify the imagery is an important factor in the resultant accuracy and aesthetic quality of the orthophoto. If a 50m regular grid elevation model was used to rectify a 10cm resolution image of an urban area from 1:3,000 scale photography, it is highly likely that the grid would fail to identify sufficient changes in relief to accurately correct the height displacement caused by changes in relief. This would result in inaccuracies of scale and would be most noticeable if adjacent images contained a linear feature such as railway that ran along an embankment. The linear feature could be distorted badly and may even result in a step between images. It is therefore essential that the following information is known: Scale of source aerial photography Camera calibration data What medium was scanned (negatives, diapositives, contact prints) Information regarding the scanner used and the resolution of the scanned image Ground control information Orientation and rectification solution The source and quality of the DEM/DTM used to rectify the imagery If in doubt, a TSA member company would be pleased to advise on this subject. When describing the photo scale of digital cameras and orthophotos this is now expressed in terms of pixel resolution. The table below shows the relationship between photo scale and pixel resolution for common mapping scales, for photogrammetric mapping. The pixel resolution can also be described as the ground sample distance (GSD), which is the distance between pixel centers measured on the ground. For example, in an image with a one-metre GSD, adjacent pixels image locations are 1 metre apart on the ground. Table 1: Table relating film scale, digital imagery resolution Photo Scale GSD Mapping Scale Price Scaling 1:3000 4cm 1:500 1: cm 1:1250 1: cm 1:2500 Note: Other scales may be used for specialist products. Some Clients & Providers may have their own specific standards, which can exceed the commonly accepted industry standards. Table extracted from the Royal Institute of Chartered Surveyors (RICS) Vertical aerial photography and derived digital imagery 5th edition guidance note 2010 (GN 61/2010). 7

8 Figure 6: Orthophoto and Mapping layer 7 Uses of Orthophotos Orthophotos can be used in a variety of applications from simple backdrops for GIS systems and illustrations to being used to create other value added products such as mapping derived from interpretation of the orthophotos. Below is a list of uses for orthophotos: Photogrammetric Mapping (Figure 6 above) Interpretation for Vector Mapping Urban Planning Asset Management Vegetation Mapping Land Use/Land Cover Mapping Change Detection Route Planning Area and length measurements 8 Tendering Considerations When procuring ortho photographs the following information should be provided: Purpose of the survey Expected accuracy of the deliverable products Information on the survey environment & specific requirements, such as surveying at specific times if tidal zone information is required or there are requirements to capture vegetation canopies etc. 8

9 A plan with the survey area outlined Specification including any special requests Deliverable data formats Access and security arrangements (if necessary for the ground control or a ground truthing survey) 9 Tender Returns A company practiced in aerial surveys and photogrammetric mapping can turn around a quotation quickly and as a client you should insist on the following details to be included in your quotation, as a minimum: A start time for the survey work to commence A delivery time of the survey data The survey Company s terms and conditions; so you understand the procedures the company uses during the course of the project A provision in the fee for a weekly (or daily) progress report; so you can allow for any changes in the program and effectively plan resources accordingly. Additional information such as method statements and risk assessments can be requested and should be supplied if required. Contact the UK Survey Association phone or visit the TSA website to locate an established survey firm. 10 Summary Digital orthophotography is a very useful product in itself, and they are also a valuable input dataset from which other datasets and products such as photogrammetric mapping can be derived from. It is important to understand how the orthophotos are to be used, and what accuracies are required for the end product derived from the orthophotos. You only need to look on Google Earth now to understand how far developed and advanced the creation and use of digital aerial photography has come in 10 or so years. The process of commissioning and successfully completing an orthophotographic survey is straight forward and simple if following the above guidelines. One key element is to use a survey company that has experience with this type of work and is a member of the TSA. 9

10 The Survey Association Formed in 1979 as The UK Land and Hydrographic Association, TSA is now established as the representative organisation for UK private surveying firms. The Association s aims are: > To provide a vehicle for members to act effectively together on agreed courses of action > To promote the interests of the profession to all those who determine the economic and social conditions in which the industry operates > To identify and represent the views of the industry. Using a TSA member By using a TSA member you can be assured that your project will get off to the best possible start. Whatever the size of project, you can be certain that TSA member companies are expert in the provision and management of spatially related data on which to base your concept, design and construction. Professional attention from a TSA surveyor will reduce risk, repetition, possibly save you money and will ensure that your project receives the best possible attention. TSA Contact Details If you would like any more information about the TSA or its members or about other Information leaflets then please contact Rachel Tyrrell at: The Survey Association Northgate Business Centre, 38 Northgate, Newark-on-Trent, Nottinghamshire NG24 1EZ Tel: office@tsa-uk.org.uk Website: Document Revision History Issue 1 August 2008 Original document Issue 2 September 2014 Revised version 10

11 Glossary of Terms TERM B/W CIR GNSS GSD LiDAR NIR RGB EXPLANATION Black and white imagery Colour Infrared imagery, Red, Green and Near Infrared channels Global Navigation Satellite System, a satellite based navigation system with global coverage Ground Sample Distance Light Detection and Ranging is an optical remote sensing technology that can measure the distance to, or other properties of target by illuminating the target with light, often using pulses from a laser Near Infrared, a non-visible portion of the electromagnetic spectrum just beyond visible red light, particularly useful if vegetation health is of interest Red, Green and Blue visible channels 11