## 2 Materials and Methods
### Datasets Used  
Production of the coSIF data product requires both observational and auxiliary datasets.  
_Observational Datasets: OCO-2 SIF and XCO\({}_{2}\)_  
OCO-2 is a NASA satellite that launched in July 2014 and actively observes two main quantities: SIF and XCO\({}_{2}\)[3; 33]. Currently, OCO-2 yields the finest spatial resolution of all spaceborne SIF observations [13], and it is often used as a benchmark for the validation of SIF data products (e.g., [14; 20; 24]). The OCO-2 satellite has a sun-synchronous, 98.8-min polar orbit with roughly a 13:36 local crossing time and a 16-day ground-track repeat cycle [34]. Observations are made in one of three modes: \"nadir\", \"glint\", and \"target\". In nadir mode, the viewing zenith angle (VZA) is near zero, and the instrument views the ground directly beneath the satellite, while the glint mode generally has higher VZAs as it tracks high reflectance from Earth's surface. Target mode has VZAs that can vary substantially, since various locations on Earth's surface are targeted when in this mode. Each mode operates at a fine spatial resolution; for example, swath widths are approximately 10 km with eight measurements across the track, each about 1.3 km \(\times\) 2.25 km (across \(\times\) along track) in nadir mode [33].  
For use among the broader scientific community, NASA publishes OCO-2 datasets as collections of \"Lite\" files. Lite files contain bias-corrected, daily Level 2 (non-gridded) data at the same spatio-temporal resolution as that of the original observations. We obtained version 10r Lite files for SIF [35] and XCO\({}_{2}\)[36], which are available through NASA's Goddard Earth Sciences Data and Information Services Center (GES DISC), each from September 2014 to February 2022 inclusive. SIF measurements are retrieved in Watts per square meter, per steradian, and per micrometer (hereafter W m\({}^{-2}\) sr\({}^{-1}\) um\({}^{-1}\)) at wavelengths of 757 nm and 771 nm. From these wavelengths, estimates of SIF are produced at a wavelength of 740 nm. As recommended in [13], we use the SIF estimates at 740 nm, which have a lower error overall than the measurements at the 757 nm and 771 nm wavelengths [3]. Further, we only retain observations associated with quality flags 0 and 1 for the Level 2 SIF data. With regard to the Level 2 XCO\({}_{2}\) data, which are available in parts per million (ppm), we only retain those associated with quality flag 0. Estimates of measurement error standard deviations accompany both the SIF measurements and the XCO\({}_{2}\) measurements; we square these measurement error standard deviations to obtain the corresponding measurement error variances.  
_Auxiliary Dataset: MODIS LCC_  
From 2001, the Terra and Aqua combined Moderate Resolution Imaging Spectroradiometer (MODIS) Land Cover Climate Modeling Grid (MCD12C1) provides yearly land-cover classification (LCC) at a 0.05-degree spatial resolution over the globe. We obtained MCD12C1 Version 6.1 [37] for 2021 to produce a binary mask of land-based prediction locations. Following the International Geosphere-Biosphere Programme (IGBP) classification scheme, we use values not marked as \"water\" to create the land mask over North America.