This process step and all subsequent process steps were performed by the same person, Robert Welk, in ArcMap (ver. 10.6.1) using tools from ArcToolbox, unless otherwise stated. For complex operations, names of specific tools used are given in CAPITAL letters (any critical parameters used are given in parentheses, separated by a semicolon, immediately after the tool name). The input and output file names are provided in [square brackets] when necessary. Units for length and area calculations are meters (m) and square meters (m2) unless otherwise stated.
a) Set the data frame coordinate system and projection to NAD 1983 UTM Zone 18N. Prepare elevation dataset to be clipped to the boundaries polygon. MOSAIC TO NEW RASTER(Pixel type=32 bit float; Mosaic operator=Mean; Cellsize=1 m) the entire NED raster to a new raster dataset with 1 m resolution [elev_mosaic.tif].
b) Calculate values for each marsh unit. DISSOLVE(Input features=[CMU_ELI]; Dissolve field=FID_CMU; Statistics field=ATOT_M2 and FLG with Statistics type= First) to dissolve the vegetated and unvegetated classes in each marsh unit. FLG flags anomalous marsh units based on absence of vegetation, elevation and surface area, and is set to (-1) no vegetated area; (-10) marsh unit elevation higher than the 99.8 percentile; (-100) marsh unit elevation less than 0.2 percentile; (-1000) marsh unit surface area less than 900 m2; and (0) no flag. Combination of negative values indicate a combination of flags. For example, -1001 indicates no vegetated area and area less than 900 m2. PROJECT(Input coordinate system=WGS 1984 Web Mercator Auxiliary Sphere; Output coordinate system=NAD 1983 UTM Zone 18N; Geographic transformation=WGS 1984 (ITRF00) to NAD 1983) the feature dataset to obtain dissolved marsh units [mu_diss.shp].
c) Clip the raster with the polygon features by using EXTRACT BY MASK to obtain [elev_idw_mu.tif].
d) ZONAL STATISTICS AS TABLE(Feature Zone Data=[mu_diss.shp]; zone field=FID_CMU; Value Raster=[elev_idw_mu.tif]; Output Table=mu; Statistics type=MEAN)
e) ADD JOIN(Layer name=[mu_diss.shp]; Input join field=FID_CMU; Join Table=mu; Output join field=FID_CMU) to marsh units layer based on the "FID_CMU" field
f) Export dataset as a shapefile [mu_elev.shp] and keep the 'mu_MEAN" and "mu_ATOT_M2" fields from the join operation.
g) Calculate elevation of the vegetated areas only. SELECT BY ATTRIBUTES FROM [CMU_ELI] where "CLASS"= 'vegetated'. Follow steps (c) to (f) using the selected features to export [veg_elev.shp].
h) JOIN FIELD(Input table=[mu_elev.shp]; Input join field=mu_FID_CMU; Join table=[veg_elev.shp]; Output join field=vg_FID_CMU; Join fields=vg_AVEG_M2,vg_ATOT_M2,vg_MEAN). Change the field names to VG_ELEV and MU_ELEV respectively for the fields with the mean elevation for the vegetated area only and mean elevation for the entire marsh unit. Set VG_ELEV to -9999 if there is no vegetated area in the marsh unit. Name AVEG_M2 and ATOT_M2 for vegetated and total area, respectively, and export to a new shapefile. [mu_elev_ELI.shp].
i) Calculate metrics that quantify the distribution of marsh unit elevation values. SKWNSS is the measure of skewness of the elevation distribution in the vegetated part of a marsh unit. Positive skewness values, i.e. right-skewed distributions, indicate clustering of elevation distributions towards lower elevations. Negative numbers indicate a left-skewed distribution and clustering towards higher elevations. LOW3RD is the percent of elevation values that fall within the lowest third of the elevation range within the vegetated part of a marsh unit. SKWNSS and LOW3RD parameters are calculated by applying the formulation of Raposa et al. (2016) to the elevation raster within the vegetated part of each marsh unit.
j) SELECT BY ATTRIBUTES FROM [CMU_ELI] where "CLASS"= 'vegetated', and join field "APGN_M2" from CMU_ELI.shp on the feature dataset [mu_elev_ELI.shp].
k) Calculate ratio of hydroflattened area in each marsh unit. Reclassify NED elevation raster with RASTER CALCULATOR (expression: SetNull('elev_mosaic.tif' == -1, 'elev_mosaic.tif') to get [elev_null.tif]. ZONAL STATISTICS AS TABLE (Input table=[mu_elev.shp, Zone field=FID_CMU], Input value raster=elev_null.tif, Statistics type=Mean) to get [hfar_mu]. ADD FIELD to output table 'MU_HFAR' and use FIELD CALCULATOR ((1-MEAN/2)). JOIN FIELD(Input Table=mu_elev.shp, Input Join Field=FID_CMU, Join Table=hfar_mu, Output Join Field=FID_CMU). If 'MU_HFAR' > 0.25, set 'mu_elev' = 0.
l) Calculate ratio of hydroflattened area in the vegetated area of each marsh unit. ZONAL STATISTICS AS TABLE (Input table=[veg_elev.shp, Zone field=FID_CMU], Input value raster=elev_null.tif, Statistics type=Mean) to get [hfar_veg]. ADD FIELD to output table 'VG_HFAR' and use FIELD CALCULATOR ((1-MEAN/2)). JOIN FIELD(Input Table=mu_elev.shp, Input Join Field=FID_CMU, Join Table=hfar_veg, Output Join Field=FID_CMU). If 'MG_HFAR' > 0.25, set 'vg_elev' = 0.
m) Rearrange field names and change the projection for better performance of web services with online base maps. PROJECT(Input coordinate system=NAD 1983 UTM Zone 18N; Output coordinate system=WGS 1984 Web Mercator Auxiliary Sphere; Geographic transformation=WGS 1984 (ITRF00) to NAD 1983) the feature dataset to obtain the final conceptual marsh units [mu_elev_ELI.shp].
Reference:
Raposa, K. B., Wasson, K., Smith, E., Crooks, J. A., Delgado, P., Fernald, S. H., et al. (2016). Assessing tidal marsh resilience to sea-level rise at broad geographic scales with multi-metric indices. Biological Conservation, 204, 263–275.
https://doi.org/10.1016/J.BIOCON.2016.10.015
