HAP - Your Questions Answered
- Input imagery refers to the positive digital image file (after scanning). Both uncomressed TIFF and PCI PIX files are supported. Note: A standard naming convention should be applied. This helps to ensure that the correct metadata is associated with the correct input image. For example: RollNumber-ImageNumber.tif, ProjectID_ImageNumber.tif, etc.
- Camera characteristics are similar (i.e. similar focal length, dimensions)
- Same Fiducial mark locations (i.e. corner, edge, both)
- Near continuous coverage
For the HAP system to operate properly, the following should be considered as a minimum amount of metadata in your file(s):
- Approximate Scene Center Coordinates
- Camera Focal Length
- Physical Paper Dimensions
- Approximate Flying Altitude
Reference Data can be any imagery used for GCP collection, road vector layers used for GCP collection and a Digital Elevation Model (DEM) used for GCP collection and orthorectification. Before choosing your reference data, it is important to ask yourself the following questions regarding its suitability:
- Can an appropriate control image be obtained that covers the project area(s)?
- Can road vector layers be obtained for the project area(s)?
- Can an appropriate DEM be obtained for the project area(s)?
For roll-film, scanners will need a specific attachment to hold and advance the spools. This attachment is usually provided with the scanner or purchased as an accessory. For flat-bed scanners, ensure the scanner is large enough to hold your film or print.
- The scanning bed dimensions should be sufficient to fit the valid image area and the surrounding boarder
- The border-areas of image prints are important as they may contain fiducial marks or information about the sensor (e.g. focal lengths, scale, etc.)
- Common aerial image print dimensions are 9”x9”, 10”x10” and sometimes larger. Similarly rolls will be 9” or 10” spools of varying length.
Scanning resolution or dots per inch (DPI) follows the general rule that the higher the DPI, the more detail your scanned images will retain, but the scanning time per print and file-size will increase. It is important to determine a sufficient DPI that ensures the required detail from the image print is obtained, but also takes into account scanning speed and file size.
- Scanning with a DPI that is set too high (i.e. 1000dpi) may be counterproductive as the detail of the image prints do not require that level of scan resolution
- From discussion with industry partners, 600dpi is a very common scan resolution used for historical aerial imagery. However, in some cases a higher resolution is required (i.e. 1000dpi).
- Please note, that contrary to common belief the DPI you scan at will not change the ground resolution (GSD) of the image, but will impact the number of pixels in the digital image file and therefore, the file size itself.
- It may not seem important at first to consider whether your scanned images will be 300mb or 800mb, but if you have a large archive of images, file storage may be a problem.
Speed is a production concern (how many frames do you expect to scan an hour, etc). Scanners specifications with indicate how many seconds it takes to scan a file of a given size and at what dpi. Please make sure to select a scanner or service that will allow you to meet your quota and product specification
Photo alignment on the scanner bed is important to consider. It is important that the image prints are placed as square as possible and that there is some level of consistency to the placement of the images when scanning. This helps with automating the fiducial mark collection during HAP processing.
- It is not important that the images be placed perfectly square as fiducial mark collection ensures that the image boundaries are correctly defined when processing with the HAP system.
A common concern is whether it is better to purchase and operate a scanner, or hire a scanning service to produce the input images for the HAP system. The following points provide a background to understanding of the HAP ecosystem and airphoto scanning in general:
- If purchasing a scanner, costs vary greatly between new and used hardware, and between regional markets. Trade publications often have sections dedicated to the purchase and sale of used equipment.
- Three scanner models that are quite reliable and suitable for HAP are Leica (DSW700), GTBI (Vexcel UltraScan 5000) and Wehrli & Associates (RM-6 Auto Scanner).
- It is important to look for a scanner that can produce digital files from your specific input media, which may be:
- Color or black & white images
- Transparent or opaque media
- Positive or negative images
- Continuous tone or line art
- Cut-sheet, single-frame or film-roll
- Consider what post-scan editing, if any, you will need to do. When deciding between purchasing a scanner vs hiring a scanning service. Post-scan editing often includes the following:
- Histogram analysis
- Contrast, brightness or gamma correction
- Merging or splitting of color channels
- Resampling the pixel size
- Debanding and dodging
- Generating image pyramids
- Post-scan processing is another service to be taken into consideration that is often offered by scanning specialists and may include image repair such as:
- Image cleaning and dust removal
- Scratch removal and correction
- Consider that shipping the film may require a bonded and insured carrier and operator, and this is the usual method. Because of the size and sensitivity of photogrammetric scanners, on-site work or transport of the hardware is not possible for short-term projects.
- For imagery that pre-dates metric cameras (e.g. no fiducials and typically black & white), a photogrammetric scanner may not be required. A smaller, simpler scanner could be used at lower cost with greater flexibility, but post-scan editing and preparation is more expensive, including tasks such as location of center-coordinates, scale and orientation. Accuracy and quality will also be lower and attention needs to be taken with respect to the requirements of the final product.