Freshwater/Saltwater Interface Estimator

How the estimate is returned

A prepared Cloud Optimized GeoTIFF (COG) combines three bands on the same 1,000-meter grid. It is published as a hosted tiled imagery layer. When you click the map, Map Viewer supplies the pixel values to an Arcade popup expression, which interprets the classification and formats the elevation and depth.

The elevation, depth, and response class are prepared beforehand. The popup reads these values; it does not interpolate a new geologic surface or calculate depth from a live terrain service.

Depth below land surface = land-surface elevation − reference-surface elevation. Preparation uses compatible units and vertical references. A negative reference elevation is below NAVD88; it does not mean a negative depth.

What the responses mean

• Class 1: estimated 10,000 mg/L TDS surface elevation and depth.
• Class 2: fresh to the base of the system; numbers describe the aquifer-system base, not a located salinity interface.
• Classes 3 and 4: the relevant interface or base source elevation is unavailable.
• Class 255: outside applicability, within the raster footprint. Beyond the footprint, the raster may provide no popup.

A stored numeric sentinel of −9999 is displayed as “Unavailable,” never as a real elevation or depth. Some cells have a valid reference elevation but lack a depth value.

A method that can serve other surfaces

The same pattern can make published aquifer tops, geologic contacts, unit thicknesses, or other regional surfaces easier to explore. Prepare aligned numeric and classification bands, document their units and meaning, publish the imagery, and use a popup to explain the result at a location. Geological and hydrological review remains essential when selecting surfaces and defining where they apply.

Use nearest neighbor resampling for this mixed numeric-and-classification raster so that discrete response codes are preserved. Keep the query layer enabled for popups, reduce its visual prominence with transparency, and disable popups on contextual layers if the estimator should be the only click response.

The popup expression

This is Arcade, the expression language used inside ArcGIS popups. Map Viewer performs pixel retrieval; the expression accesses the raw band array through $pixel["Raster.ServicePixelValue.Raw"]. Explicit numeric conversion handles values supplied as text.

The expression below was retrieved from the published estimator map. In Map Viewer, use Pop-ups → Add content → Arcade → Edit expression and replace the starter snippet with the full expression.

Download the Arcade expression

var values = $pixel["Raster.ServicePixelValue.Raw"];

if (IsEmpty(values)) {
    return {
        type: "text",
        text: "<p>No raster values are available at this location.</p>"
    };
}

if (Count(values) < 3) {
    return {
        type: "text",
        text: "<p>The estimator requires three raster bands.</p>"
    };
}

var elevation = values[0];
var depth = values[1];
var code = Number(values[2]);

function feet(value) {
    if (IsEmpty(value)) {
        return "Unavailable";
    }

    var numericValue = Number(value);

    if (IsNan(numericValue) || numericValue == -9999) {
        return "Unavailable";
    }

    return Text(numericValue, "#,##0.0") + " ft";
}

var html = "";

if (code == 1) {
    html =
        "<p><b>Estimated 10,000 mg/L TDS surface</b></p>" +
        "<p>Elevation: <b>" + feet(elevation) +
        "</b> relative to NAVD88" +
        "<br>Depth below land surface: <b>" +
        feet(depth) + "</b></p>";
} else if (code == 2) {
    html =
        "<p><b>Fresh to the base of the Floridan aquifer system</b></p>" +
        "<p>The regional interpretation indicates water below " +
        "10,000 mg/L total dissolved solids through the mapped " +
        "aquifer system.</p>" +
        "<p>Base-of-system elevation: <b>" +
        feet(elevation) + "</b> relative to NAVD88" +
        "<br>Depth to base of system: <b>" +
        feet(depth) + "</b></p>" +
        "<p>These values describe the aquifer-system base; " +
        "they do not locate a freshwater/saltwater interface.</p>";
} else if (code == 3) {
    html =
        "<p><b>Interface estimate unavailable</b></p>" +
        "<p>This location is in the mapped interface region, " +
        "but the source interface raster has no elevation value.</p>";
} else if (code == 4) {
    html =
        "<p><b>Fresh-to-base interpretation; base estimate unavailable</b></p>" +
        "<p>The regional interpretation indicates fresh-to-base " +
        "conditions, but the source aquifer-base raster has no " +
        "elevation value at this location.</p>";
} else if (code == 255) {
    html =
        "<p><b>Outside the estimator's area of applicability</b></p>" +
        "<p>No interface or aquifer-base estimate is provided.</p>";
} else {
    return {
        type: "text",
        text:
            "<p>Unexpected classification value: " + Text(code) +
            ". Check the imagery resampling settings before " +
            "using this result.</p>"
    };
}

html +=
    "<hr>" +
    "<p><b>How to interpret this estimate</b></p>" +
    "<p>The mapped boundary represents 10,000 mg/L total " +
    "dissolved solids (TDS). The freshwater/saltwater title " +
    "refers to this threshold.</p>" +
    "<p>Source: USGS Professional Paper 1807, Figure 53, " +
    "and Data Series 926, Figure 54, with derived service rasters.</p>" +
    "<p>Depth = land-surface elevation minus reference-surface " +
    "elevation, using compatible vertical datums and units. " +
    "The displayed depth was calculated during raster preparation.</p>" +
    "<p>Values represent a 1,000-meter grid cell. Depth uses " +
    "the source depth raster sampled at the cell center. " +
    "Decimal formatting does not indicate measurement accuracy.</p>" +
    "<p>This regional mapping estimate does not establish " +
    "site-specific water quality or conditions in an individual well.</p>";

return {
    type: "text",
    text: html
};

Scale, sources, and interpretation

Values represent regional mapping on a 1,000-meter grid. Displayed depth represents sampling at the grid-cell center, rather than a precise-location terrain measurement. Decimal formatting does not imply measurement accuracy. The estimator is not a determination of water quality in an individual well or a site-specific engineering or regulatory assessment.

The Apalachicola classification COG is not included in this three-band query product. Any original cartographic context retained in the map is separate from the estimator response.

• USGS Professional Paper 1807, Figure 53: regional framework and mapped TDS boundary.
• USGS Data Series 926, Figure 54 and associated derived service rasters.
• Esri Arcade imagery popup element documentation.