DOI: 10.5281/zenodo.2653629
title: Spectrometer measurements of snow and bare ground targets and simultaneous measurements of snow conditions

description: The dataset holds measurements of reflected sunlight spectrum from snow and different snow free targets (moss, lichen, rock surface, ground vegetation). The purpose of the measurements has been to create a dataset to better understand the effect of snow cover characteristics and different bare ground targets on reflected sunlight, and to relate this information to observed at satellite reflectances from snow covered and partially snow covered boreal forests. The instrument in the measurements was Field Spec Pro JR from Analytical Spectral Devices Inc. Additionally to the 237 bands measured between 350-2500 nm, the dataset contains the following parameters from the measurement sites: id, location, time of measurement, target type (snow, bare ground etc.), cloudiness (in octas), snow depth (cm), snow temperature at 5 cm (degree C), snow temperature half way through the snow pack (degree C), soil temperature (degree C), air temperature (degree C), average grain size (mm), grain type, snow water content (subjective), snow patchiness (%) in the surrounding area, visible impurities in snow yes/no and land cover.
description: -Sampling-

The data has been collected in field measurement campaigns in years 2010 to 2011. The emphasis on timing of the measurements has been on the spring melting season, where, from satellite imaging perspective, there are mixed reflectance signatures from snow and bare ground. The measurements were conducted using ASD Field Spec Pro JR (Analytical Spectral Devices) spectrometer with a spectral range from 350 to 2500 nm sampled with 237 bands. Measurements were carried out with a tripod, the sensor looking at nadir. The diameter of the measurement spot was 19 cm. The measurements were mainly conducted at Sodankylä Arctic Research Station of the Finnish Meteorological Institute (FMI), Finland, but small campaign was also done in Espoo, Finland. The site coordinates are given in the data files. The measurements of snow reflectance are associated with snow properties measured from a snow pit dug at the site. The following physical properties of snow were measured: snow depth (cm); snow temperature in two levels and soil temperature (degree C); grain size (mm, by comparing the grains against mm scale grid); grain type (simple classification); estimate of snow patchiness in the surroundings (%), wetness of snow (after Colbeck 1990, now Fierz 2009) and impurities in snow pack (clean vs. clearly seen impurities). Information is also provided from prevalent weather conditions: cloud cover (in octas), air temperature (degree C, measurement with portable thermometer). Additionally, information is provided on: time, date, site location, land cover type. 

Colbeck, S. C., Akitaya, E., Armstrong, R. L., Gubler, H., Lafeuille, J., Lied, K., McClung, D. M. & Morris, E. M. (1990) The International Classification for Seasonal Snow on the Ground. International Commission on Snow and Ice (IAHS), World Data Center A for Glaciology, University of Colorado, Boulder, CO, USA. 

-Processing-

A single reflectance spectrum entry in the dataset contains a mean from 30 measurements for each band, and their associated standard deviation. Cases with variable lighting conditions during the measurements have been filtered out from the dataset. The data is affected by illumination conditions, namely the sun elevation, which appears as messy spectrum in the NIR-spectrum around 1300-1500nm. Furthermore, the longer wavelengths from around 1800nm onwards is also highly variable, but we leave it to the user to filter out the data as best seen for the purpose. Further details on the processing can be found in the associated documentation.

-Quality control-

During the data collection each measurement (of 30 spectra) is calibrated by measuring known reflectance from a Spectralon (R) optical standard reflectance panel. Also the illumination conditions are measured (solar irradiance) at the start of the measurement at one site. 4-5 closely spaced targets (with the assumption that the surface conditions remain the same) of same type are measured at each site. The wavelength area around 1400 and 1900 nm are affected by atmospheric water absorption. These effects have not been filtered out from the data. Further information is provided in the associated documentation. The instrument used has been cross-calibrated in laboratory conditions with two other ASD Field Spec Pro JR's and one Analytical Spectral Devices (ASD) FieldSpec HandHeld spectrometer. There is a document (in finnish) on the cross-calibration provided with the data.

Reflectance_spectra_2010_2011_average.csv: The file contains average reflectance from 30 samples for one measurement. Each measurement can be connected to the corresponding field data through the measurement ID. The table is arranged with reflectance measurements on rows and site id's on columns. 

Reflectance_spectra_2010_2011_standard_deviation.csv: The file contains the corresponding standard deviations of the spectrum averages, calculated from the 30 spectrum samples for each site. The wavelengths are on rows and the site id's on columns.

Reflectance_spectra_2010_2011_measurement_site_characteristics.csv: The file contains columns with following information from each measurement: 
ID = Measurement ID
Lat = Geographic latitude WGS-84
Lon = Geographic longitude WGS-84
Date = Date of measurement YYYYMMDD
Hours in Eastern European Time (EET, in summer EEST)
Minutes in Eastern European Time (EET, in summer EEST) 
TargetType =  Measured target type in the field of view of the sensor (1=snow, 2=snow in tree trunk shadow, 3=snow in tree branch shadow, 4=open ground after snow melt, 5=lichen, 7=moss)
Cloudiness = Visually estimated cloundiness (octas, 0/8 clear sky, 8/8 full cloud cover)
SnowDepth = (cm) measured with a ruler
SnowTemp5cm =  Snow temperature at 5 cm depth from the surface (degree C)
SnowTempHalf = Snow temperature measured from the middle of the snowpack (degree C)
SoilTemp = Temperature measured from the snow-soil interface (degree C)
AirTemp = Air temperature (degree C), measured in 1 m height from surface
GrainSize (mm, estimated with grain plate and from digital photographs)
GrainType = Snow grain type classes (1=fine separated crystals, 2=metamorphosed separated crystals, 3=clustered crystals,4=almost slush,5=slush, 6=ice layer) 
SnowWater = Rough snow wetness estimate (1=dry,2=moist, 3=wet,4=very wet, 5=slush, based on snowball test)
SnowPatch=Snow patchiness visually estimated (%, no snow=100)
Impurities = Impurities in snow visually estimated (no=1,yes=2)
LandCover = Land cover/ land use type of the measurement area (1=open(heath), 2=open mire, 3= pine forest, 4=spruce forest, 5=mixed forest (coniferous dominant), 6=sparse forest, 7=fell, 8=snow on lake ice, 9=grassland/agricultural field)

No Data Value = -99