Flatten Elevation is not smooth

Dear all,
I need some suggestion about flatten elevation.
This is my source code to add flatten elevation layer to Earth.

    Map*         map = m_mapNode->getMap();
    OGRFeatureOptions flattenData;
    flattenData.name() = "flatten";
    flattenData.url() = FLATAREA_URL;
    flattenData.buildSpatialIndex() = true;

    FeatureElevationOptions options;
    options.featureOptions() = flattenData;
    options.attr() = "ELEVATION";
    options.profile() = ProfileOptions("global-geodetic");

    ElevationLayer *layer = new ElevationLayer(options);
    layer->options().minLevel() = 7;
    layer->options().maxLevel() = 17;
    layer->setName("Flatten");
    layer->options().terrainPatch() = true;
    layer->options().cachePolicy() = CachePolicy::NO_CACHE;
    map->addLayer(layer);

The result is here:



The flatten elevation is not smooth.
I change size tile of layer, but it's not effective.
Please help me.
Thanks you.

long pham,

It is working as designed. The layer will not smooth between the flattened area and the terrain.

Thanks Glenn Waldron,
I want to flatten an area to place model (example buildings, airport runways ...). But my elevation data is not detail, I need to flatten it. How can i do it with osgearth? I already try to using FlatteningLayer, but it is not success.