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# Depth and Sonar Sentences
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Depth and sonar sentences provide water depth measurements from various types of depth finding equipment including echo sounders and sonar systems.
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## Depth Measurement Sentences
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### DBT - Depth Below Transducer
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```python { .api }
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class DBT(TalkerSentence):
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    """Depth Below Transducer."""
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    depth_feet: str          # Depth in feet
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    unit_feet: str           # 'f' for feet
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    depth_meters: str        # Depth in meters
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    unit_meters: str         # 'M' for meters
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    depth_fathoms: str       # Depth in fathoms
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    unit_fathoms: str        # 'F' for fathoms
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```
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### DBS - Depth Below Surface
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```python { .api }
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class DBS(TalkerSentence):
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    """Depth Below Surface."""
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    depth_feet: str          # Depth in feet
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    feets: str               # 'f' for feet
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    depth_meter: str         # Depth in meters
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    meters: str              # 'M' for meters
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    depth_fathoms: str       # Depth in fathoms
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    fathoms: str             # 'F' for fathoms
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```
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### DPT - Depth of Water
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```python { .api }
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class DPT(TalkerSentence):
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    """Depth of Water."""
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    depth: str               # Water depth in meters
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    offset: str              # Offset from transducer
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    range: str               # Maximum range scale
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```
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## Usage Examples
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```python
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import pynmea2
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# Parse depth below transducer
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msg = pynmea2.parse("$SDDBT,12.3,f,3.7,M,2.0,F*35")
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print(f"Depth: {msg.depth_feet} feet = {msg.depth_meters} meters = {msg.depth_fathoms} fathoms")
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# Depth: 12.3 feet = 3.7 meters = 2.0 fathoms
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# Parse depth below surface
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msg = pynmea2.parse("$SDDBS,15.8,f,4.8,M,2.6,F*09")
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print(f"Surface depth: {msg.depth_meter} {msg.meters}")  # 4.8 M
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# Parse water depth with offset
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msg = pynmea2.parse("$SDDPT,4.2,0.5,*4F")
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print(f"Water depth: {msg.depth}m, Offset: {msg.offset}m")  # 4.2m, 0.5m
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if msg.range:
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    print(f"Range scale: {msg.range}")
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# Create depth sentence
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dbt_data = ['12.3', 'f', '3.7', 'M', '2.0', 'F']
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depth_msg = pynmea2.DBT('SD', 'DBT', dbt_data)
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print(str(depth_msg))  # $SDDBT,12.3,f,3.7,M,2.0,F*35
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```
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## Unit Conversions
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Common depth unit conversions:
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- **1 fathom = 6 feet = 1.8288 meters**  
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- **1 meter = 3.28084 feet**
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- **1 foot = 0.3048 meters**
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```python
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import pynmea2
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msg = pynmea2.parse("$SDDBT,12.3,f,3.7,M,2.0,F*35")
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# Convert between units manually
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feet_to_meters = float(msg.depth_feet) * 0.3048
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meters_to_feet = float(msg.depth_meters) * 3.28084
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fathoms_to_meters = float(msg.depth_fathoms) * 1.8288
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print(f"Conversions:")
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print(f"  {msg.depth_feet} ft = {feet_to_meters:.2f} m")      # 12.3 ft = 3.75 m
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print(f"  {msg.depth_meters} m = {meters_to_feet:.2f} ft")    # 3.7 m = 12.14 ft  
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print(f"  {msg.depth_fathoms} F = {fathoms_to_meters:.2f} m") # 2.0 F = 3.66 m
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```