[gdal-dev] Fema Flood Maps NAV88 - proj4text

[Chris English]

Just to close this out,
New York New Jersey FEMA advisory Base Flood Elevations are ESPG:3857

> From: sglish at hotmail.com
> To: gdt at ir.bbn.com; gdal-dev at lists.osgeo.org
> Date: Sat, 2 Feb 2013 11:39:23 -0500
> Subject: Re: [gdal-dev] Fema Flood Maps NAV88 - proj4text
> 
> 
> The parcels are, of course, horizontal only datum.
> from the .prj
> 
> PROJCS["NAD_1983_StatePlane_New_Jersey_FIPS_2900_Feet",
> GEOGCS["GCS_North_American_1983",
> DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],
> PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
> PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",492125.0],
> PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-74.5],
> PARAMETER["Scale_Factor",0.9999],
> PARAMETER["Latitude_Of_Origin",38.83333333333334],UNIT["Foot_US",0.3048006096012192]]
> 
> and from a representative flood map .prj
> 
> PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",GEOGCS["GCS_WGS_1984",
> DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],
> PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
> PROJECTION["Mercator_Auxiliary_Sphere"],PARAMETER["False_Easting",0.0],
> PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0],
> PARAMETER["Standard_Parallel_1",0.0],PARAMETER["Auxiliary_Sphere_Type",0.0],UNIT["Meter",1.0]]
> 
> which seems now, looking at them, should just be a standard transform. But then this from
> Adv_MO_FloodZones.shp.xml 
> 
> -<geodetic>
> <horizdn>GCS WGS 1984</horizdn>
> <ellips>World Geodetic System 1984</ellips>
> <semiaxis>6378137.000000</semiaxis>
> <denflat>298.257222</denflat>
> </geodetic>
> </horizsys>
> <vertsys>
>     <gridsysn>NAVD88</gridsysn>
>     <plandu>FEET</plandu>
> </vertsys>
> 
> well, I'll continue to muddle and head scratch.
> ----------------------------------------
> > From: gdt at ir.bbn.com
> > To: sglish at hotmail.com
> > CC: gdal-dev at lists.osgeo.org
> > Subject: Re: [gdal-dev] Fema Flood Maps NAV88 - proj4text
> > Date: Sat, 2 Feb 2013 08:21:11 -0500
> >
> >
> > 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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