Pekka Voipio

Leaf area index estimation of boreal forest using ENVISAT ASAR

A method for retrieval of leaf area index (LAI) using ENVISAT Advanced Synthetic Aperture Radar vertical/horizontal (VV/HH) polarization ratio was derived for boreal forests. Five alternating polarization single-look complex images of the test site were acquired in summer 2003. The swath range from IS1-IS6 was studied. The VV/HH polarization ratio correlated quite well with the ground truth LAI values. The mean error of the LAI estimates was 0.27 for the test site with mixed forest when data from all images and stands were used without separating between species (or swaths). The respective mean LAI estimation error was 0.3 for Norway spruce (Picea abies (L.) Karst.) and 0.07 for Scots pine (Pinus sylvestris L.) dominated stands.

Leaf Area Index (LAI) Estimation of Boreal Forest Using Wide Optics Airborne Winter Photos

A new simple airborne method based on wide optics camera is developed for leaf area index (LAI) estimation in coniferous forests. The measurements are carried out in winter, when the forest floor is completely snow covered and thus acts as a light background for the hemispherical analysis of the images. The photos are taken automatically and stored on a laptop during the flights. The R2 value of the linear regression of the airborne and ground based LAI measurements was 0.89.

Leaf Area Index Estimation of Boreal and Subarctic Forests Using VV/HH ENVISAT/ASAR Data of Various Swaths

This paper demonstrates the potential of dual polarization synthetic aperture radar (SAR) images in the estimation of the leaf area index (LAI) of boreal forests. The SAR data do not suffer from the low sun elevation and frequent cloud cover, which often complicate the use of optical wavelengths for LAI retrieval in the area. The analysis was based on a large number of environmental satellite (ENVISAT) advanced synthetic aperture radar (ASAR) alternating polarization vertical polarization (VV)/horizontal polarization (HH) single-look-complex images covering several test sites, boreal and subarctic, during summers 2003-2006. The swath range from IS1 to IS7 was studied. In all test sites, a linear regression with the VV/HH backscattering ratio as the independent variable could typically be used for the estimation of LAI. All swaths could be used for the estimation, but larger incidence angles generally performed better. The best results were obtained for swath IS6. The swaths could be used also together, but better results were obtained using the diverse swaths individually. The LAI estimation error decreased essentially exponentially with the number of pixels averaged to give one backscattering value. The LAI estimation accuracy for a set of average quality ASAR images of swath IS6 reached 0.1 when the averaging number of pixels was 33 150, which would correspond to an area of about 2.2 km2 for images with no overlap. The spatial LAI estimation error decreased with the number of images covering the same area.

Airborne Estimation of Boreal Forest LAI in Winter Conditions: A Test Using Summer and Winter Ground Truth

A recently developed airborne method for estimation of leaf area index (LAI) in coniferous forests is used for comparing the LAI values in summer and winter conditions. The airborne measurements based on a wide-optic camera are carried out in winter when the forest floor is completely snow covered and thus acts as a light background for the image analysis. The photographs are taken automatically and stored on a laptop during the flights. The R2 value of the linear regression between the airborne and ground-based LAI measurements was 0.97 for all plots. Despite the unfavorable weather conditions, the average difference between the ground-based and airborne regression-based LAI estimates was 0.08, and in 90% of the cases, it was smaller than 0.13. The corresponding relative differences were 14% and 23%. The standard deviation of the ground-based LAI values measured within a plot was, on the average, of the same order. The winter-time values of the LAI of coniferous trees were estimated to be 24% smaller than the preceding summer-time values.

Boreal forest LAI retrieval using both optical and microwave data of ENVISAT

A LAI retrieval method based on microwave and optical data of ENVISAT is developed. The estimation accuracy of the combination method is better than for either data type alone. Keywords-component; leaf area index; boreal forest; ENVISAT; remote sensing

Estimation of boreal forest LAI in winter conditions: Test of a new method using wide optics airborne images

A new simple airborne method based on wide optics camera was developed for leaf area index (LAI) estimation in coniferous forests. The measurements are carried out in winter, when the forest floor is completely snow covered and thus acts as a light background for the images analysis. The photos are taken automatically and stored on a laptop during the flights. The R2 value of the linear regression of the airborne and ground based LAI measurements was 0.90 for all plots. Despite of the unfavourable weather conditions the average difference of the ground based and airborne regression based LAI estimates was 0.16 and in 80% of cases it was smaller than 0.28. The standard deviation of the plotwise ground based LAI values was of the same order.

]JERS-1 SAR data in forest biomass mapping in northern taiga zone

Forest biomass mapping was studied in a site in northern Finland (latitude 66deg). Two JERS SAR scenes that were acquired in a dry period were used. Locally derived linear regression models between forest stem volume and L-band SAR amplitude produced correlation coefficients of 0.7. An earlier regression model, which had been derived in a site in southern Finland, produced under-estimates of approximately 50 m3/ha in a study site where stem volume varied between 0 and 220 m3/ha.