[gdal-dev] Fema Flood Maps NAV88 - proj4text

[Greg Troxel]

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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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