To assess the current topography of the tidal marshes we conducted survey-grade elevation surveys at all sites between 2009 and 2013 using a Leica RX1200 Real Time Kinematic (RTK)Global Positioning System (GPS) rover (±1 cm horizontal, ±2 cm vertical accuracy; Leica Geosystems Inc., Norcross, GA; Figure 4). At sites with RTK network coverage (San Pablo, Petaluma, Pt. Mugu, and Newport), rover positions were received in real time from the Leica Smartnet system via a CDMA modem (www.lecia-geosystems.com). At sites without network coverage (Humboldt, Bolinas, Morro and Tijuana), rover positions were received in real time from a Leica GS10 antenna base station via radio link. When using the base station, we adjusted all elevation measurements using an OPUS correction (www.ngs.noaa.gov/OPUS). We used the WGS84 ellipsoid model for vertical and horizontal positioning. We verified rover accuracy and precision by measuring positions at local National Geodetic Survey (NGS) benchmarks and temporary benchmarks established at each site (Table 1). Average measured vertical errors at benchmarks were 1-2 cm throughout the study, comparable to the stated error of the GPS. At each site, we surveyed marsh surface elevation along transects oriented perpendicular to the major tidal sediment source, with a survey point taken every 12.5 m; 50 m separated transect lines. We used the Geoid09 model to calculate orthometric heights from ellipsoid values (m, NAVD88; North American Vertical Datum of 1988) and projected all points to NAD83 UTM zone 10 or zone 11 using Leica GeoOffice (Leica Geosystems Inc, Norcross, GA, v. 7.0.1).We synthesized the elevation survey data to create a digital elevation model (DEM) at each site in ArcGIS 10.2.1 Spatial Analyst (ESRI 2013; Redlands, CA) with exponential ordinary kriging methods (5 x 5 mcell size) after adjusting model parameters to minimize the root-mean-square error (RMS). We used elevation models as the baseline conditions for subsequent analyses in this study including tidal inundation patterns, SLR response modeling, and mapping of sites by specific elevation (flooding) zones.