Nikon D70 – Geospatial Modeling & Visualization

Good Photos vs. Bad Photos for Close-range Photogrammetry

adam — Thu, 31 Jan 2013 16:37:42 +0000

“Good” close-range photogrammetry example from Ostia Antica, Italy. Note that the object (the temple) is framed tightly, and that all objects (both near and far) are in sharp focus.

When it comes to close-range photogrammetry, the difference between “good” photos and “bad” photos can be the difference between getting useful 3D information and having complete failure. There are many different variables contributing to success or failure of a project, but to help avoid the most common mistakes a photographer can follow the general guidelines outlined below.

Basic photographic concepts that, when followed, generally produce acceptable digital images:

Camera/lens Properties:
-Use a mid to high resolution camera (at least 12-15MP)
-Use a fixed (non-zoom) lens
-Tape the focus ring (and set to manual focus)
-If using a zoom lens, tape the zoom ring and use one focal length for the entire project

Camera Placement:
-Use a tripod and stable tripod head
-Frame the subject tightly, making use of the entire sensor area
-Maintain 60-80% overlap between photos
-Ensure all important areas of the object are visible in at least three images
-Be aware of camera geometry required by software (baseline, convergent angles)

Camera Settings:
-Use aperture priority mode (set to between f/8 and f/16)
-Use a timer or wired/wireless shutter release to minimizes motion blur
-Use mirror lock-up, if available, to further minimizes motion blur

A list of common mistakes made while capturing digital images for a close-range photogrammetry project:

Camera/lens:

“Bad” close-range photogrammetry example. Note that the object (the temple) is not framed tightly, and that most objects are blurry and out of focus.

-Use of low resolution camera (8MP or less)
-Changing zoom (focal length) between images
-Use of loose/damaged lens
-Significant re-focusing due to varying distance from object

Camera placement:
-Handheld camera (no tripod)
-Insufficient overlap between images
-Inefficient use of sensor area (too far from subject)
-weak camera geometry (multiple images from one position, short baseline, overall poor network of image locations/orientations)

Camera settings:
-shallow depth of field (below f/8)
-manual shutter release (causes motion blur)

Checklist for Close-Range Photogrammetry Image Collection

adam — Tue, 24 Jul 2012 18:34:48 +0000

Download a printable checklist in PDF format here.

[wptabs effect=”slide” mode=”vertical”] [wptabtitle] Before you leave the office[/wptabtitle] [wptabcontent]

– Check that the battery is charged (check spare battery if taking one)
– Copy images from past projects to a laptop if needed and format the card
– Attach the lens you plan to use
– Check the lens for excessive dust

[/wptabcontent]
[wptabtitle] Goals for each image you capture[/wptabtitle] [wptabcontent]

– Use entire frame
– Sharp focus at all distances
– Good exposure throughout image

[/wptabcontent]
[wptabtitle] Camera setup for typical close-range project with DSLR[/wptabtitle] [wptabcontent]

– Set to Aperture Priority mode
– Set aperture to between f8 and f16 (depending on DOF¹ needed)
– Set the camera to collect RAW and/or JPEG Large
– Configure other settings as needed
– Mount camera to tripod and frame the object for the first image
– Focus the camera on the object (using auto or manual), then turn the lens to manual focus and tape the focus ring so that it doesn’t move
– If using a zoom lens, tape the zoom so that it doesn’t move
– Set the camera to use a 2 second timer (or use wired shutter release)
– Set camera to use mirror lock-up (to avoid camera vibration)

[/wptabcontent]
[wptabtitle] Notes[/wptabtitle] [wptabcontent]
¹ DOF stands for Depth of Field and is a term used to describe the depth of the scene that is in focus. Smaller apertures create more depth of field, though, at some point a small aperture will introduce blur due to diffraction. Learn more here at the dpreview.com glossary.[/wptabcontent] [/wptabs]

Camera Calibration Documentation for Close-Range Photogrammetry

adam — Thu, 19 Jul 2012 15:37:51 +0000

Camera Calibration Photo

Each camera (referring to a camera and lens combination) used in a close-range photogrammetry project should be calibrated and a calibration file should be saved with the project data. Some close-range photogrammetry software does not require this information before processing the image block; however, it will provide most of this information after processing. The tables below describe what is appropriate to document for this process. Download a printable form in PDF format here or in a spreadsheet (.xlsx) format here.

For each camera/lens calibration:

Element	Description
Camera/Lens name	Name used to identify this camera/lens combination (e.g. CAST5DmkII-2_28mm).
Date of calibration	Date that the calibration was performed.
Camera calibration file name	The exact file name for the camera calibration.
Camera specifics	Specific make and model of the camera and lens used.
Array dimensions in pixels	Width and height of digital array measured in pixels.
Array dimensions in mm	Width and height of digital array measured in millimeters.
Focal length	As indicated by camera calibration.
Principal Point	As indicated by camera calibration.
Lens distortions (K1, K2, P1 and P2 parameters)	Radial and decentering distortion parameters as measured by the camera calibration.
Affine distortions	Affine distortion parameters as measured by the camera calibration (if measured).
Calibration Quality Values	Quality values such as overall RMS, maximum residual, and photo coverage (%) from the calibration process.
Calibration adjustment report	A report on the quality of the camera calibration including the correlation between exterior and interior parameters.

For each calibration image (if performing a camera calibration with a target):

Entry	Description
Image file names	File names for calibration images.
Calibration target description	Include target dimensions, creator and description.

Image Acquisition Documentation for Close-Range Photogrammetry

adam — Thu, 19 Jul 2012 15:27:55 +0000

Nikon D70

Organization and documentation during image collection in the field is especially important. 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.

For each group of images:

Entry	Description
Project name	The project name or name for the dataset.
Number of images	Total number of images.
File name for planimetric sketch or map	File name and extension. Should include outline of subject and surrounding objects (if any), indicated location and orientation of each image (using a “V” symbol to indicate orientation), and other special comments and/or observations.
Camera calibration file	Reference to the camera calibration file if available.
Additional notes	Any additional notes the surveyor feels applicable. Could list images containing control and/or scaling references.
For each image:
Image file name	File name and extension.
Textural description of location and orientation	Should describe general location (e.g. north side) and camera to subject orientation (e.g. view to south).
Format conversions (if any)	List of format conversions performed on the digital images and the software used.

Project Level Documentation for Close-Range Photogrammetry

adam — Thu, 19 Jul 2012 14:37:34 +0000

A number of guides and documentation standards for the collection and processing of close-range photogrammetric data (or in some cases for photographic documentation in general) have been published by various organizations. This form and other metadata forms on this site follow the Archaeology Data Service (ADS) Guides to Good Practice. See the list of helpful webpages and publications for a link to the ADS guides and other helpful links.

Every CRP project should contain project level documentation, including a description of the project and site, date(s) of the survey, name(s) and organization(s) of the surveyor(s), and other useful notes. The table below describes the appropriate documentation for this process and you can download a printable form in PDF format here or in a spreadsheet (.xlsx) format here.

Element	Description
Title	The project name or name for the dataset.
Description	The original purpose for the survey work.
Subject	Keywords for the subject content of the dataset (qualified using e.g. the English Heritage NMR Monument Type Thesaurus or the MDA Object Type Thesaurus).
Coverage	Site location and description. The address, or coordinates for the site and a description of the site and object or structure to be surveyed. Coverage should also include any relevant period terms.
Creators	Full name and organization(s) of the surveyor(s)
Identifiers	Project or reference numbers (e.g. HABS/HAER/HALS survey number) used to identify the dataset.
Dates	Date or dates that the survey was conducted in both the field and/or lab.
Intended accuracy or scale of the survey	The originally intended accuracy or scale that the survey was to achieve.
Make & model of camera(s) and lens(es)	Detailed make and model of the camera and lens used for the survey. List the associated calibration files if applicable.
Description of reference information used	Describe any existing reference information available to the surveyor(s), including paper plans or digital spatial data of the site or object.
Additional project notes	Any additional project notes that the surveyor feels applicable.