[wptabs style=”wpui-alma” mode=”vertical”] [wptabtitle] A BASIC INTRODUCTION [/wptabtitle]

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Why is pre-processing necessary?

For most close-range photogrammetry projects digital images will need to be captured in a RAW format, preserving the maximum pixel information which is important for archival purposes. Therefore it will likely be necessary to do some pre-processing in order to convert RAW images into a file format accepted by the photogrammetry software being used for the project.

If a color chart or gray card was using during image capture, it may also be useful to perform a white balance on the image set. There are a number of tools/software packages available for this purpose, but below we will describe a potential workflow using Adobe products for batch processing.

Overall steps of this workflow:

–  Batch convert RAW to DNG (Adobe DNG Converter)
–  Batch white balance (Camera Raw)
–  Batch image adjustments (Camera Raw)
–  Batch save to JPEG (or TIFF) format (Camera Raw)

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[wptabtitle] BATCH CONVERT RAW DATA [/wptabtitle] [wptabcontent]

Batch RAW to DNG with Adobe Digital Negative (DNG) Converter Software

As an open extension of the TIFF/EP standard with support for EXIF, IPTC and XMP metadata, the Adobe DNG format is rapidly becoming accepted as a standards for storing raw image data (primarily from digital photography).

For more information about file formats for archival, see the Archaeological Data Service (ADS) Guides to Good Practice.

Steps to Batch Convert:

1. Download and install Adobe DNG Converter. As of the date this workflow was published, version 7.2 of Adobe DNG Converter is a free tool available for download on the Adobe website.

Adobe DNG Converter is a free tool available for download on the Adobe website.

2. This tool converts an entire folder (aka batch) of images at one time. Use the tool interface to select the appropriate input folder containing the RAW images.

3. If needed, use the interface to design a naming scheme to be used for the new file names.

4. Set preferences for compatibility (e.g. Camera Raw 5.4 and later) and JPEG Preview (e.g. medium size). As an option, you can embed the original RAW file inside the new DNG files. This will, or course, increase the file size of the new DNG file.

5. Click “Convert” to start the process. Wait for this to finish.

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[wptabtitle] BATCH WHITE BALANCE – 1 [/wptabtitle] [wptabcontent]

Batch white balance, image processing, and exporting with Adobe – Part 1: Adobe Bridge

It is considered best practice to (correctly) use a quality color chart or gray card when capturing digital images for any CRP project.  Performing a white balance for each image set (or each lighting condition) can dramatically enhance the appearance of a final product (i.e. ortho-mosaic). This particular workflow uses Adobe Bridge and the Adobe Camera Raw tool, but a similar process can be done in other (free) software as well.

Adobe Bridge – Open in Camera Raw

1. Open Adobe Bridge and navigate to the folder containing the digital images (DNG files).

2. Select the appropriate images (including images with color chart/gray card).

3. Use the “File” menu to select “Open in Camera Raw”

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[wptabtitle] BATCH WHITE BALANCE – 2 [/wptabtitle] [wptabcontent]

Batch white balance, image processing, and exporting with Adobe – Part 2 : Camera Raw tool

4. Camera Raw will open and all of the selected images will appear on the left side of the window. Select the image with the color chart/gray card you would like to use for white balancing and other adjustments. Do all adjustments to this one image. We will apply the same changes to all images in the following slide ‘Batch Image Adjustment’.

Adobe Camera Raw – Image Processing Settings

5. By default, Camera Raw may attempt to apply a number of image processing settings that you should remove. This can be done using the interface on the right hand side of the screen. Check that all settings (with the exception of Temperature and Tint, which are set by the white balance tool in the next step) are set to zero. Be sure to check under each of the tabs.

6. Select the “Color Sampler Tool”  found in tool bar at the top of the window and:

A. If using a color chart, add a color sample inside the black and white squares. After adding these you should see the RGB pixel values for each sample.

B. If using a gray card, add a color sample inside the gray portion of the card.

7. Select the “White Balance Tool” from the tool bar at the top of the window and click on the gray portion of the chart (or card) to apply a white balance. At the same time, notice how the RGB values of the color sample(s) change. The RGB values should not differ by more than five or six (e.g. the white sample could be R: 50, G: 50, B: 51). If they differ by too much there could be a problem with the white balance. Try clicking a slightly different spot in the gray portion of the chart.

8. If other adjustments need to be made (i.e. exposure, brightness, contrast) make them now.

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[wptabtitle] BATCH IMAGE ADJUSTMENTS [/wptabtitle] [wptabcontent]

Applying adjustments to all

Once the white balance and adjustments have been made to this one image, we can apply the same to all the other images open in Camera Raw.

To do this, click “Select All” in the top left corner of the window – then click “Synchronize.” Wait for this to finish.

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[wptabtitle] BATCH SAVE TO JPEG OR TIFF [/wptabtitle] [wptabcontent]

Saving

Once the Synchronization is complete, click the “Save Images” in the bottom left corner of the window (make sure all images are still selected). The “Save Options” dialog allows you to choose a folder for the images to be saved to, a naming scheme, a file extension and format, and a quality/compression. Choose the settings you prefer and click “Save.”

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[wptabtitle] CONTINUE TO… [/wptabtitle] [wptabcontent]

Continue to PhotoScan – Basic Processing for Photogrammetry

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3D View of DEM Overlaid With 3D Breaklines

Close Range Photogrammetry:

Typical deliverables created as the end result of a CRP project could include 2D vector graphics (planimetric or elevation type CAD drawings), dense point clouds, 3D polylines, facetized models (mesh) of an object or surface, and raster graphics such as rectified or fully orthorectified images. Each deliverable created should include appropriate metadata for each of the above mentioned steps, as well as metadata for the additional processing performed to create the final file.

Because the diversity of possible end products and software packages associated with photogrammetry, it is difficult to specify the metadata required. We recommend using the ADS Guides to Good Practice to identify the appropriate metadata for your specific file. Below is a list of ADS Guides and the file types they should be used for:

2D CAD drawings and 3D vectors: http://guides.archaeologydataservice.ac.uk/g2gp/Cad_Toc
3D point clouds and facetized models (mesh): http://guides.archaeologydataservice.ac.uk/g2gp/LaserScan_Toc
Raster images: http://guides.archaeologydataservice.ac.uk/g2gp/RasterImg_Toc

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Low Altitude & Aerial Photogrammetry:

A discussion and comparison of file export formats is currently being developed. Please check back soon! And in the meantime, please see the ADS Guides to Good Practice regarding Aerial Survey for detailed information regarding documentation and standards.

Aligned Images in PhotoScan

In order to extract three dimensional points from two dimensional images, it is necessary to perform a triangulation with at least two images (a stereo pair). When more than two images are used in a triangulation, we refer to the group of images as a ‘block’. To perform a triangulation, we must measure a sufficient number of tie, control, and/or check points throughout the block. Constraints may also be placed on certain sets of points to enforce angular, linear, and/or planar properties. Once a triangulation is successful, image exterior orientation parameters (along with estimate for accuracy) should be available to the user. These are important pieces of information for downstream deliverables and should be documented. The table below describes the appropriate documentation for this process. Download a printable form in PDF format here or in a spreadsheet (.xlsx) format here.

[wptabs mode=”vertical”] [wptabtitle] Project Planning[/wptabtitle] [wptabcontent]

Project planning includes a number of important considerations that will influence the success of the project. These include developing a strategy for the site/object, selecting the equipment and software to be used, calibrating equipment if needed, obtaining any required permissions, and starting the documentation process.[/wptabcontent]

[wptabtitle] Image and Control Acquisition[/wptabtitle] [wptabcontent]

There are a number of strategies for the collection of images in a photogrammetric project. Typically, the strategy is driven by the software used to process the images, and, more specifically, whether the type of processing requires a stereo or convergent (see figure below) set of images. This topic should be covered in the help file or manual for the software you plan to use.

Stereo (left) vs Convergent (right) image pairs

External control information can be added to a photogrammetric project for two reasons: 1) to position the model relative to a datum and/or 2) to provide geometric constraints on the photogrammetrically derived model. If the photogrammetric model is to be situated partially or wholly within an existing reference frame or datum (geodetic, mapping or local) then sufficient external references defined in this frame must be integrated into the project. A 3D reference frame or datum is defined by scale, position and orientation. Typically, reference information is in the form of control points (photo-identifiable points with known coordinates in a reference frame), lengths of photo-identifiable objects, and/or angles between photo-identifiable objects.

Adding Control Points in PhotoScan Pro

The minimum amount of information needed to scale, position and orient a photogrammetric model is two 3D control points and one 1D control point. If more than minimal control is provided (e.g. three or more 3D control points) then the control information could be used to help define the shape of the photogrammetric model as well as define its datum. In this case, the surveyor must ensure that the control information is, as a rule-of-thumb, at least 3x more accurate than the photogrammetric model itself. If it is not, then the control information will distort the photogrammetric model and potentially have deleterious effects on its relative accuracy.

It is also possible to apply control after a 3D model (mesh or other 3D CAD) has been created. In this case the control will only serve to position the model in space and will not cause distortion.[/wptabcontent]

[wptabtitle] Image Processing and Block Triangulation[/wptabtitle] [wptabcontent]

Most digital images captured in the field will require some digital processing, which include white balancing or any adjustments to the brightness, contrast, or other common image properties. One important note is to never crop (or change the height/width in any way) an image intended for photogrammetry.

In order to extract three dimensional points from two dimensional images, it is necessary to perform a triangulation with at least two images (a stereo pair). When more than two images are used in a triangulation, we refer to the group of images as a ‘block’. In order to perform a triangulation of the entire block (known as a bundle block adjustment), the user must measure a sufficient number of tie, control, and/or check points throughout the block. Constraints may also be placed on certain sets of points to enforce angular, linear, and/or planar properties. After a successful bundle adjustment, the user can extract and export 2D and/or 3D products.
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[wptabtitle] Creating and Exporting Deliverables[/wptabtitle] [wptabcontent]

Typical deliverables created as the end result of a CRP project could include 2D vector graphics (planimetric or elevation type CAD drawings), dense point clouds, 3D polylines, facetized models (mesh) of an object or surface, and raster graphics such as rectified or fully orthorectified images. Each deliverable created should include appropriate metadata for each of the above mentioned steps, as well as metadata for the additional processing performed to create the final file.[/wptabcontent]

[wptabtitle] Workflow Chart[/wptabtitle] [wptabcontent]This workflow provides a graphic overview of the steps involved in a Close-Range Photogrammetry Project. Click the image to see a larger version.

Basic Workflow of CRP Project

[wptabs mode=”vertical”] [wptabtitle] In Office Setup: Begin the Metadata Process[/wptabtitle] [wptabcontent]

1. Choose a method for documenting the project (e.g. paper, laptop)
2. Fill in known project metadata items (e.g. project name, date of survey, site location, etc.)
3. Create a sketch map of the area (by hand or available GIS/maps)

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[wptabtitle] Plan project and prepare equipment[/wptabtitle] [wptabcontent]

1. Develop a project plan based on desired end product and required accuracy (review CRP steps if needed)
2. Decide what equipment will best suite the project (e.g. what camera? what lens? method for white balance?)
3. Print and check off items for close-range photogrammetry checklist
4. Print other applicable metadata forms and have ready

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[wptabtitle] In Field Setup: Configure Camera Settings[/wptabtitle] [wptabcontent]

1. Photo quality (RAW and JPEG Large)
2. White balance (custom or auto)
3. Auto rotate (off)
4. Live View function (stills only)
5. Mirror lock up (enabled) *recommended for shutter speeds longer than 1/30th
6. Self-timer (2 or 10 second)
7. Turn off any image stabilization features

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[wptabtitle] In Field Setup: Set Focus[/wptabtitle] [wptabcontent]

1. Though some projects may require an auto focus for each image, in an ideal project the focal length (and therefor focus) of the lens will remain constant throughout a CRP image set.
   1. For architectural photogrammetry, or other projects with a distance of more than 10 meters between the camera and object, find the switch on the lens with “AF” and “MF” and switch to MF (Manual Focus), then manually rotate and secure the focus ring to infinity
      

      Focus Ring

   2. For projects with a shorter, constant distance (e.g. all photos will be captured from 6 meters), autofocus the lens for the first photo, then switch the lens to manual focus and secure that focus setting for all remaining photos
   3. For projects with a shorter, varying distance (e.g. photos will be captured anywhere from 2-5 meters), use autofocus for all images
2. Turn the Mode Dial to “Av” (Aperture value) mode
3. Set aperture to appropriate value (between F8 and F16)
4. Take test photo and check for sharpness throughout the object of interest
5. Adjust settings as necessary

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[wptabtitle] Taking the Photos: Setting Up[/wptabtitle] [wptabcontent]

1. Have documentation materials ready
   1. As you collect images, document the process according to ADS
   2. Make sure you capture a color checker chart, gray card, or other means of white balancing the images in the lab
2. Choose appropriate photo location
   1. Obstructions between the camera and object should be minimized
   2. The tripod should be stable and should not move throughout the image capture

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[wptabtitle] Frame and Take the Photo[/wptabtitle] [wptabcontent]

1. Frame the Photo 
   1. Adjust the tripod head to position the aim of the camera
   2. The object should fill the frame
2. Press the shutter button
3. Pressing the button should initiate the mirror lockup, followed by the timer and image capture
4. You should hear one thump, a series of beeps, and then the shutter
   1. Review the image for sharpness and exposureset.
      1. By default, the camera will display the image for a few seconds
      2. If the LCD goes black, push the Playback button to review
      3. Push the Magnify button to zoom in, and pan across the image
      4. Be sure to review objects both near and far for sharpness
      5. Look at the histogram and make sure the brightness values are well distributed and not clipped

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   [wptabtitle] Reconsider your Settings[/wptabtitle] [wptabcontent]

   1. If the image is not sharp make sure you’re choosing appropriate aperture 
      1. Is your aperture too large (meaning the f/ number is small)? Maybe try one or two stops smaller (e.g. from f/10 to f/16)
      2. Did the tripod move during image capture? With small apertures, you can expect long exposure times. Be sure the tripod remains 100% motionless during the entire exposure
      3. Did you have an extremely long exposure? More than 10-20 seconds? This could indicate you aperture is too small (e.g. f/22 or a higher number) for the amount of available light, or you are suffering from diffraction as light passes the aperture. Stop down closer to f/16 and try again

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   [wptabtitle] If the image is sharp…[/wptabtitle] [wptabcontent]Move to the next location (according to your strategy) and repeat.[/wptabcontent]

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