Hemispherical image taken at Chowilla, South Australia.
IMPORTANT NOTE: Hemispherical canopy photographs are best collected in diffuse lighting conditions, when the sky is uniformly grey (eg. around dawn and dusk when the sun is below the horizon, or on days with 100% cloud cover). The quality of these images for analysis purposes is compromised if taken at other times, particularly around solar noon. If it is only possible to take pictures during sunlight conditions, it is possible to mask out the influence of the sun, but this is not ideal and will bias the gap fraction estimates. Try not to have fluffy clouds in the images either, as the brightness of clouds can reduce leaf cover at the edges of the leaves. In addition, hemispheric photos should not be taken in windy conditions.
Plot selection process
The hemispheric photo collection process outlined in this document assumes the photos are taken at intervals along established star transects. Sites should be selected in the manner outlined in the star transects protocol.
Description of plot layout
Star transect plots are 100 m in length (50 m radius). 13 photos are taken (1 in the centre of the star transect, 6 half way along each arm ( at the 25 m point) and 6 at the ends of each transect.
Basic data required
The following data must be recorded for data management purposes:
Geographic coordinates (easting, northing,zone or longitude, latitude) at plot centre
Operators: who collected the data
Date: consistent format (dd/mm/yyyy)
Start Time: hh:mm when transect started
End Time: hh:mm when transect completed
Plot name: name of specific plot, if taken in conjunction with star transects, use that reference
Photo number for last photo at each point
Azimuth of camera top (0 for top of camera at north)
infrared vs RGB
Height photos taken at in cm
Data collection process
Field equipment checklist
3 x 100 m tape measures
Camera with hemispheric lens, preferably a circular fisheye lens rather than a full-frame fisheye lens
Tripod or monopod
GPS – preferably differential
Lay out plot
This protocol is designed for data to be collected in conjunction with SLATS star transects. As such the plot should already exist. If data is being collcted independently, then layout plot in the manner outlined here.
Take a GPS reading at the centre of the plot.
It is best to use survey-grade differentially corrected GPS if possible because it is important that the gaps within and between trees lines up with the fractional cover estimated from LiDAR. The horizontal error from a handheld GPS can be in the range of of 2 m to 10 m and can cause problems when validating high spatial resolution products, such as airborne LiDAR. If ground control points are available that will be identifiable in both the field and the imagery, these can be used to improve the integration of field and image data through a distance and bearing measurement between the GCP and the centre of the plot.
Record site details on field sheet.
Record the following details on the field sheet:
Field operator names/initials
Camera and lens details (including if infrared or simple)
Height photos taken at
Orientation of camera to north
Photo format (raw or jpg)
Use a separate sheet for each height/lens combination.
Take photos at the centre location, then halfway along each of the plots six arms. If the vegetation is sparse or tall, take a further photo at the end of each arm.Make sure all required photo details (photo numbers, height and time) are recorded on the field sheet in the appropriate location.
Ensure the following settings are used for the photographs:
Exposure - Set up the camera so that it takes 3 photos at each point (2 f-stops below automatic exposure, 1 f-stop below automatic exposure and at automatic exposure). This can be achieved with bracketing starting at -1.
Format - Set the camera to take the photos in raw format
DPI - Try to make the dpi as high as possible (e.g. 8mp). If photos are taken in raw format, the DPI will default to the camera's maximum.
Level camera -Make sure that the camera is level. It is important that the camera is within 10 degrees of horizontal. If available, use a tripod with a bubble to level the camera. A monopod can also be used, but a tripod is preferable for balance.
Alignment - Align the camera so that magnetic north is aligned with the top of the photograph. Make a note of the alignment if there is any variation to this.
Camera height - Images should be taken at the appropriate level for the site. Typically this is at breast height (1.3 m above the height of the ground surface). However, if there is an understory, it is good to have photos above the understory and below it (e.g. at 2 m - if that is the height of the understory, and 0.5 m). Also, if branches extend to the ground (like at the jack pine sites), then it is good to take the photo from as close to the ground as possible to get most of the biomass.