Atmospheric deposition of nitrogen and sulfur causes many deleterious effects on ecosystems including acidification and excess eutrophication.  Assessments to support development of strategies to mitigate these effects require spatially and temporally continuous values of nitrogen and sulfur deposition.   Members of NADP’s Total Deposition Science Committee (TDEP) developed an approach for estimating dry deposition using ambient concentration data from air monitoring networks such as the Clean Air Status and Trends Network (CASTNET) and NADP’s Ammonia Monitoring Network (AMoN), and modeled dry deposition velocities and flux data from the Community Multiscale Air Quality (CMAQ) model.  These dry deposition values are then combined with wet deposition values from the NADP National Trends Network (NTN) to estimate values of total deposition of sulfur and nitrogen.  Although this approach presumably reduces the overall error due to estimating ambient concentrations at monitoring locations compared with using modeled values alone, additional uncertainty is introduced at interstitial spaces due to interpolation error and other assumptions in the approach.  This talk will summarize efforts to quantify the components of this added uncertainty and hopefully provide guidance in further refinements of the TDEP approach.