Add altitude threshold for aircraft drawing

Explain `constants.py` in README

README.md

@@ -49,3 +49,14 @@ secrets = {
     "opensky_password": "YOUR_OPENSKY_PASSWORD"
 }
 ```
+
+### Constants
+A collection of functionality-related constants is specified in `constants.py`, which can be adjusted to suit your needs:
+
+| Variable Name              | Description                                   | Default  |
+|----------------------------|-----------------------------------------------|----------|
+| `MAP_CENTER_LAT`           | Map center latitude, decimal degrees          | `42.41`  |
+| `MAP_CENTER_LON`           | Map center longitude, deimal degrees          | `-71.17` |
+| `GRID_WIDTH_MI`            | Map grid width, miles                         | `15`     |
+| `SKIP_GROUND`              | Skip drawing aircraft on the ground           | `True`   |
+| `GEO_ALTITUDE_THRESHOLD_M` | Skip drawing aircraft below this GPS altitude | `20`     |

constants.py

@@ -1,3 +1,8 @@
+# Mapping information
 MAP_CENTER_LAT = 42.41
 MAP_CENTER_LON = -71.17
 GRID_WIDTH_MI = 15
+
+# Aircraft plotting
+SKIP_GROUND = True
+GEO_ALTITUDE_THRESHOLD_M = 20

skyportal/displaylib.py

@@ -7,6 +7,7 @@
 from adafruit_bitmapsaver import save_pixels
 from adafruit_display_text import label
 
+from constants import GEO_ALTITUDE_THRESHOLD_M, SKIP_GROUND
 from skyportal.aircraftlib import (
     AIRCRAFT_ICONS,
     AircraftIcon,
@@ -170,7 +171,8 @@ def draw_aircraft(
         aircraft: list[AircraftState],
         default_icon: AircraftIcon = BASE_ICON,
         custom_icons: dict[int, AircraftIcon] = AIRCRAFT_ICONS,
-        skip_ground: bool = True,
+        skip_ground: bool = SKIP_GROUND,
+        geo_altitude_threshold_m: float = GEO_ALTITUDE_THRESHOLD_M,
     ) -> None:
         """
         Clear the currently plotted aircraft icons & redraw from the provided list of aircraft.
@@ -192,6 +194,10 @@ def draw_aircraft(
                 n_ground += 1
                 continue
 
+            if ap.geo_altitude_m is None or ap.geo_altitude_m < geo_altitude_threshold_m:
+                n_ground += 1
+                continue
+
             # If we've gotten here then lat/lon can't be None
             icon_x, icon_y = calculate_pixel_position(
                 lat=ap.lat,  # type: ignore[arg-type]