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Environmental monitoring networks such as the Integrated Carbon Observation System (ICOS) in Europe, the National Ecological Observatory Network (NEON) in the U.S., or the Terrestrial Ecosystem Research Network (TERN) in Australia deploy different sampling schemes for in situ measurements. We report on the intercomparison of measurements of the canopy gap fraction with different digital hemispherical photography setups adopting ICOS, NEON, and TERN sampling schemes. The test was carried out at the Järvselja Radiation Transfer Model Intercomparison (RAMI) birch stand. Results show that spreading out sampling points which cover more of the plot is important for a good representation of the forest as a whole. The NEON tower plot layout scheme may be more prone to errors in overall canopy properties estimation than ICOS or TERN due to its compact sampling layout and should always be used in conjunction with its distributed plots. Different camera setups involving different camera operators, camera bodies, lenses and settings yield slightly varied results, and it is important to ensure that the images are taken in such a way that they would not be over or underexposed, or out of focus. As a conclusion we recommend always to carry out intercomparison measurements with old and new cameras when devices are upgraded. Our study contributes towards establishing the uncertainty and evaluating potential error budget stemming from collecting in situ measurements using different sampling schemes and camera setups. [ABSTRACT FROM AUTHOR]

• How to assemble a calibration board from easily affordable components. • Obtaining geometric and radiometric correction functions with a simple board. • Using a device-specific function to reproject to the standard equidistant projection. • Using a device-specific function to correct vignetting effect. • Standardized images with equidistant projection and values linearly related to radiance. Canopy hemispherical photography (HP) is a technology for vegetation surveying that usually employs consumer-oriented devices. Variations on those devices are caused by technological advances and design. These variations, despite providing the potential for more reliable surveying, make the measurements challenging. The knowledge about how to standardize devices output is available, what is missing is a simple and inexpensive methodology to accomplish it. This is needed since the use of consumer-oriented devices creates an expectation of simplicity and affordability. To fill this methodology gap, we developed an easy-to-assemble calibration board of near-zero material cost (simple board) and outlined how to use it to develop device-specific functions: the projection function (f p) and the vignetting function (f v). The former relates view zenith angle (θ) with distance from image center, while f v models the radial decreasing of brightness known as vignetting effect. Explanations are accompanied by tailored R code. Altogether, we call it "the simple method". Three devices were used to test it: a Nikon Coolpix 5700 with a FC-E9 converter (D1), a Nikon D610 with a Nikkor 8 mm fisheye lens (D2), and an iPhone 6 Plus with an Olloclip fisheye converter (D3). The reference data was taken with D1, corrected for vignetting effect with a photometric sphere and reprojected to the standard equidistant projection with a f v developed by Delta-T Devices (www.delta-t.co.uk). The root mean square error (RMSE) of the simple method were 0.55° θ and 0.021 relative radiance (R R) for D1; 1.8° θ and 0.1 R R for D2; and 2.1° θ and 0.066 R R for D3. This level of standardization will help to simplify the classification of canopy gaps, a key processing step in HP. And, if it is broadly adopted, will facilitate comparisons between independent studies and large time series. [ABSTRACT FROM AUTHOR]