What causes these geometry differences between original PostGIS data and Geoserver WFS?

Question

Why does GeoServer (v2.5.1) WFS layer (see red line/layer below) not overlay features of source PostGIS table or related WMS layer in QGIS (tested in v2.6 and 2.12)? All layers are defined in the same GCS (EPSG: 4326) (interstate Tiger shp data originally obtained from US Census site), see image below:

I've un-checked the QGIS option/setting "Enable feature simplification....". I'm guessing this may be a server side configuration for WFS/GML, or I fear a limitation with GCS served data as WFS.

Here is the store xml:

<dataStore>
  <id>DataStoreInfoImpl-xyz</id>
  <name>General</name>
  <type>PostGIS</type>
  <enabled>true</enabled>
  <workspace>
    <id>WorkspaceInfoImpl--xyz</id>
  </workspace>
  <connectionParameters>
    <entry key="port">thePort</entry>
    <entry key="Connection timeout">20</entry>
    <entry key="passwd">xyz</entry>
    <entry key="dbtype">postgis</entry>
    <entry key="encode functions">false</entry>
    <entry key="namespace">theName</entry>
    <entry key="schema">public</entry>
    <entry key="create database">false</entry>
    <entry key="fetch size">10000</entry>
    <entry key="preparedStatements">false</entry>
    <entry key="min connections">1</entry>
    <entry key="host">localhost</entry>
    <entry key="validate connections">true</entry>
    <entry key="max connections">20</entry>
    <entry key="Support on the fly geometry simplification">false</entry>
    <entry key="database">theDB</entry>
    <entry key="Loose bbox">true</entry>
    <entry key="Expose primary keys">false</entry>
    <entry key="Max open prepared statements">50</entry>
    <entry key="Estimated extends">true</entry>
    <entry key="user">theUser</entry>
  </connectionParameters>
  <__default>false</__default>
</dataStore>

Here is the featuretype xml:

<featureType>
  <id>FeatureTypeInfoImpl-xyz</id>
  <name>layerName</name>
  <nativeName>layerName</nativeName>
  <namespace>
    <id>NamespaceInfoImpl--xyz</id>
  </namespace>
  <title>layerName</title>
  <keywords>
    <string>features</string>
    <string>layerName</string>
  </keywords>
  <nativeCRS>GEOGCS[&quot;WGS 84&quot;, &#xd;
  DATUM[&quot;World Geodetic System 1984&quot;, &#xd;
    SPHEROID[&quot;WGS 84&quot;, 6378137.0, 298.257223563, AUTHORITY[&quot;EPSG&quot;,&quot;7030&quot;]], &#xd;
    AUTHORITY[&quot;EPSG&quot;,&quot;6326&quot;]], &#xd;
  PRIMEM[&quot;Greenwich&quot;, 0.0, AUTHORITY[&quot;EPSG&quot;,&quot;8901&quot;]], &#xd;
  UNIT[&quot;degree&quot;, 0.017453292519943295], &#xd;
  AXIS[&quot;Geodetic longitude&quot;, EAST], &#xd;
  AXIS[&quot;Geodetic latitude&quot;, NORTH], &#xd;
  AUTHORITY[&quot;EPSG&quot;,&quot;4326&quot;]]</nativeCRS>
  <srs>EPSG:4326</srs>
  <nativeBoundingBox>
    <minx>-123.393768310547</minx>
    <maxx>-67.7812576293945</maxx>
    <miny>25.7490768432617</miny>
    <maxy>49.0023765563965</maxy>
    <crs>EPSG:4326</crs>
  </nativeBoundingBox>
  <latLonBoundingBox>
    <minx>-123.393768310547</minx>
    <maxx>-67.7812576293945</maxx>
    <miny>25.7490768432617</miny>
    <maxy>49.0023765563965</maxy>
    <crs>GEOGCS[&quot;WGS84(DD)&quot;, &#xd;
  DATUM[&quot;WGS84&quot;, &#xd;
    SPHEROID[&quot;WGS84&quot;, 6378137.0, 298.257223563]], &#xd;
  PRIMEM[&quot;Greenwich&quot;, 0.0], &#xd;
  UNIT[&quot;degree&quot;, 0.017453292519943295], &#xd;
  AXIS[&quot;Geodetic longitude&quot;, EAST], &#xd;
  AXIS[&quot;Geodetic latitude&quot;, NORTH]]</crs>
  </latLonBoundingBox>
  <projectionPolicy>FORCE_DECLARED</projectionPolicy>
  <enabled>true</enabled>
  <metadata>
    <entry key="cachingEnabled">false</entry>
  </metadata>
  <store class="dataStore">
    <id>DataStoreInfoImpl-xyz</id>
  </store>
  <maxFeatures>0</maxFeatures>
  <numDecimals>0</numDecimals>
</featureType>

Answer 1

The output would seem consistent with a decimal truncation, which is performed by GeoServer WFS 1.0 as configured in the global panel. Normally the value would be 8 decimals, but your precision issues seem to suggest a setting like 5 decimals instead (making the rough assumption of 100km per decimal degree, in practice that changes a lot depending on the latitude, see https://en.wikipedia.org/wiki/Latitude#Length_of_a_degree_of_latitude )

Answer 2

It's defintely related to the resolution somehow. I wouldn't know if the lower resolution is enforced by GeoServer or QGIS as I don't know these tools.

Take a look at the image below where I have added a sort of grid line layer on top of the map. Can you see how the distance between the black lines is similar horisontal and vertical?

I have only added lines where there seemed to be a node in the red line layer so in some areas the distance is doubled between the grids lines.

So the question is now where can you specify a lower resolution (precision truncation) or simplification of your data that would return a result like this?