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The issue of deriving cross-scale aggregation rules has been heavily investigated during the past two decades. The widely used approach consists of formulating grid-scale surface radiances using the same equations that govern the patch-scale behavior but whose arguments are the aggregate expressions of those at the patch-scale. Such approach derives area-averaged or effective radiative surface temperature as it might be observed using low spatial resolution satellite data. The problem however is such satellite data exhibits large directional effect and no study has addressed this issue. The present work tackles this problem. The directional effects will be studied by modeling. For that, an infrared sensor observing a 2D plane heterogeneous surface are modeled. 2D plane heterogeneous surface are simulated by a 2 homogeneous element grid (vegetation-bare soil). The angular properties of local surfaces, assumed homogeneous, are calculated according to a multiple scattering model. By applying the principle of aggregation, the equivalent angular radiance of the whole heterogeneous scene is then defined. This radiance is particularly sensitive to the directional effects, In particular when the spatial variation of surface temperature is significant and when natural surface is non-lambertian. As a result, an angular variation of 6% was obtained in directional surface between 70° zenith angle and on-nadir measurements.