Simplify the load_earth_magnetic_anomaly and load_earth_relief functions (#3203)

Co-authored-by: Yvonne Fröhlich <[email protected]>

Author: seisman

pygmt/datasets/earth_magnetic_anomaly.py

@@ -136,21 +136,20 @@ def load_earth_magnetic_anomaly(
     ...     resolution="20m", registration="gridline", data_source="wdmam"
     ... )
     """
-    magnetic_anomaly_sources = {
+    # Map data source to prefix
+    prefix = {
         "emag2": "earth_mag",
         "emag2_4km": "earth_mag4km",
         "wdmam": "earth_wdmam",
-    }
-    if data_source not in magnetic_anomaly_sources:
+    }.get(data_source)
+    if prefix is None:
         raise GMTInvalidInput(
             f"Invalid earth magnetic anomaly data source '{data_source}'. "
             "Valid values are 'emag2', 'emag2_4km', and 'wdmam'."
         )
-    dataset_prefix = magnetic_anomaly_sources[data_source]
-    dataset_name = "earth_wdmam" if data_source == "wdmam" else "earth_mag"
     grid = _load_remote_dataset(
-        name=dataset_name,
-        prefix=dataset_prefix,
+        name="earth_wdmam" if data_source == "wdmam" else "earth_mag",
+        prefix=prefix,
         resolution=resolution,
         region=region,
         registration=registration,

pygmt/datasets/earth_relief.py

@@ -139,37 +139,35 @@ def load_earth_relief(
     # resolutions of original land-only SRTM tiles from NASA
     land_only_srtm_resolutions = ["03s", "01s"]
 
-    earth_relief_sources = {
+    # Map data source to prefix
+    prefix = { 
         "igpp": "earth_relief",
         "gebco": "earth_gebco",
         "gebcosi": "earth_gebcosi",
         "synbath": "earth_synbath",
-    }
-    if data_source not in earth_relief_sources:
+    }.get(data_source)
+    if prefix is None:
         raise GMTInvalidInput(
             f"Invalid earth relief data source '{data_source}'. "
-            "Valid values are 'igpp', 'gebco', 'gebcosi' and 'synbath'."
+            "Valid values are 'igpp', 'gebco', 'gebcosi', and 'synbath'."
         )
-    # Choose earth relief data prefix
+    # Use SRTM or not.
     if use_srtm and resolution in land_only_srtm_resolutions:
-        if data_source == "igpp":
-            dataset_prefix = "srtm_relief"
-        else:
+        if data_source != "igpp":
             raise GMTInvalidInput(
                 f"Option 'use_srtm=True' doesn't work with data source '{data_source}'."
                 " Please set 'data_source' to 'igpp'."
             )
-    else:
-        dataset_prefix = earth_relief_sources[data_source]
+        prefix = "srtm_relief"
     # Choose earth relief dataset
     match data_source:
         case "igpp" | "synbath":
-            dataset_name = "earth_igpp"
+            name = "earth_igpp"
         case "gebco" | "gebcosi":
-            dataset_name = "earth_gebco"
+            name = "earth_gebco"
     grid = _load_remote_dataset(
-        name=dataset_name,
-        prefix=dataset_prefix,
+        name=name,
+        prefix=prefix,
         resolution=resolution,
         region=region,
         registration=registration,