I got this to work on a simple form in case anyone is interested.

Assuming two single line fields with ids q1 and q2, the following code will auto-populate them with the device's lattitude and longitude.

function getLocationConstant() {
    if(navigator.geolocation) {
        navigator.geolocation.getCurrentPosition(onGeoSuccess,onGeoError);  
    } else {
        $("#Field1, #Field2").val("Geolocation not supported");
    }
}

// If geolocation is successful
function onGeoSuccess(event) {
    $("#Field1").val(event.coords.latitude);
    $("#Field2").val(event.coords.longitude);
}

// If geolocation error occurs
function onGeoError(event) {
    $("#Field1, #Field2").val("Geolocation error occurred.");
}

$(document).ready(function() {
	getLocationConstant();
});

When the form is loaded, it might look something like this:

As Ken said, the user needs to explicitly allow location detection on their browser. If they don't, or if the device doesn't have location services, the fields get populated with a "geolocation not supported" message.

Edit: The browser may prompt the user to allow it to gather location information. This can present a problem when Laserfiche Forms saves a copy of the form to the repository. Because Forms does this by opening the form in a browser and generating an image, the browser prompt may still occur. If it does, it can interfere with scripts you are running on the form.