diff --git a/setup.cfg b/setup.cfg
index d4394c0..074047f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -25,14 +25,20 @@ python_version = >=3.10
 install_requires =
     numpy >= 1.13, <2.0.0
     importlib-metadata >= 3.6; python_version < "3.10"
-    xarray
-    netcdf4
-    cf-units
-    cftime
 package_dir =
     =src
 packages = pyaro, pyaro.timeseries, pyaro.csvreader
-test_require = tox:tox
+test_require = 
+    tox:tox
+    cf-units
+    xarray
+    netcdf4
+
+[options.extras_require]
+relalt =
+    cf-units
+    xarray
+    netcdf4
 
 [tox:tox]
 min_version = 4.0
diff --git a/src/pyaro/timeseries/Filter.py b/src/pyaro/timeseries/Filter.py
index 3235b9b..963c5df 100644
--- a/src/pyaro/timeseries/Filter.py
+++ b/src/pyaro/timeseries/Filter.py
@@ -6,16 +6,21 @@
 from datetime import datetime
 import inspect
 import re
+import sys
 import types
-from typing import Any
 
 import numpy as np
-import xarray as xr
-from cf_units import Unit
 
 from .Data import Data, Flag
 from .Station import Station
 
+try:
+    # Optional dependencies required for relative altitude filter.
+    import xarray as xr
+    from cf_units import Unit
+except ImportError:
+    pass
+
 logger = logging.getLogger(__name__)
 
 class Filter(abc.ABC):
@@ -886,7 +891,17 @@ def __init__(self, topo_file: str | None = None, topo_var: str = "topography", r
         -----
         - Stations will be kept if abs(altobs-altmod) <= rdiff.
         - Stations will not be kept if station altitude is NaN.
+
+        Note:
+        -----
+        This filter requires additional dependencies (xarray, netcdf4, cf-units) to function. These can be installed
+        with `pip install .[relalt] 
         """
+        if "cf_units" not in sys.modules:
+            logger.warning("relaltitude filter is missing required dependency 'cf-units'. Please install to use this filter.")
+        if "xarray" not in sys.modules:
+            logger.warning("relaltitude filter is missing required dependency 'xarray'. Please install to use this filter.")
+            
         self._topo_file = topo_file
         self._topo_var = topo_var
         self._rdiff = rdiff