Create DPT-surface.js (#102)

Using surfaceToTransducer to calculate depth

sentences/DPT-surface.js

@@ -0,0 +1,27 @@
+/**
+DPT - Depth of Water
+$--DPT,x.x,x.x*hh
+ Fields:
+ - Water depth relative to transducer, meters
+ - Offset from transducer, meters. Positive means distance from transducer to water line, negative means distance from transducer to keel
+ - Checksum
+ */
+// NMEA0183 Encoder DPT   $IIDPT,9.2,1.1*4B
+const nmea = require('../nmea.js')
+module.exports = function (app) {
+  return {
+    sentence: 'DPT',
+    title: 'DPT - Depth at Surface (using surfaceToTransducer)',
+    keys: [
+      'environment.depth.belowTransducer',
+      'environment.depth.surfaceToTransducer'
+    ],
+    f: function dpt (belowTransducer, surfaceToTransducer) {
+      return nmea.toSentence([
+        '$IIDPT',
+        belowTransducer.toFixed(2),
+        surfaceToTransducer.toFixed(3)
+      ])
+    }
+  }
+}