You may want to join the proj list. Your parcel maps are in EPSG (ESRI, really) 102711, which is:
http://spatialreference.org/ref/esri/102711/ This is a horizontal-only datum. I have only encountered horizontal data in parcel maps (Mass, not NJ). Do you have any heights presented with this? If so, do you understand what they are? If there are heights, I would expect they are in NAVD88, but they could be in ellipsoidal height relative to the GRS80 ellipsoid and the NAD83 orientation. Ellipsoidal height is relatively recent, and estimated orthometric heights based on geoid models and observed ellispoidal heights is where NGS is heading, but I would doubt most practitioners are doing that yet. Your flood maps are in EPSG:5703: http://spatialreference.org/ref/epsg/5703/ This is a vertical-only datum. Are there horizontal coordinates? If so, what are they in? I would expect some sort of NAD83, and their purpose is to locate where the heights are given. So what I think you want is to know the NAVD88 heights around the parcel of interest, so you therefore want all your horizontal coordinates in NAD83, and to leave the height in NAVD88. (Water flows downhill in NAVD88, but does not necessarily flow towards lower height values in ellipsoidal height coordinates!!) You were talking about a geoid file, which presumably transforms NAVD88 height to something, which is probably ellipsoidal height based on NAD83. That's the typical geoid model published by NGS. There are also geoid models from WGS84 ellipsoidal height, but these do not transform to NAVD88. Do you have any height data in other than NAVD88? If so, please describe it.
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