From ec62897be337fbcba6c30b944560a414e24b7d5e Mon Sep 17 00:00:00 2001
From: Alex-Lewandowski <aflewandowski@alaska.edu>
Date: Thu, 22 Jan 2026 14:55:15 -0900
Subject: [PATCH] update NISAR DEM endpoints and add informative error handling
 to fetch.download_file()

---
 src/multirtc/dem.py   |  6 +++---
 src/multirtc/fetch.py | 28 ++++++++++++++++++++--------
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/src/multirtc/dem.py b/src/multirtc/dem.py
index f071570..73fa7ca 100644
--- a/src/multirtc/dem.py
+++ b/src/multirtc/dem.py
@@ -15,7 +15,7 @@
 
 
 gdal.UseExceptions()
-URL = 'https://nisar.asf.earthdatacloud.nasa.gov/STATIC/DEM/v1.1/EPSG4326'
+URL = 'https://nisar.asf.earthdatacloud.nasa.gov/NISAR/DEM/v1.2/EPSG4326'
 EGM2008_GEOID = {
     'WORLD': [
         '/vsicurl/https://asf-dem-west.s3.amazonaws.com/GEOID/us_nga_egm2008_1.tif',
@@ -133,8 +133,8 @@ def get_dem_granule_url(lat: int, lon: int) -> str:
     lon_tens = np.floor_divide(lon, 20) * 20
     lon_cardinal = 'W' if lon_tens < 0 else 'E'
 
-    prefix = f'{lat_cardinal}{np.abs(lat_tens):02d}_{lon_cardinal}{np.abs(lon_tens):03d}'
-    filename = f'DEM_{lat_cardinal}{np.abs(lat):02d}_00_{lon_cardinal}{np.abs(lon):03d}_00.tif'
+    prefix = f'{lat_cardinal}{np.abs(lat_tens):02d}/{lat_cardinal}{np.abs(lat_tens):02d}_{lon_cardinal}{np.abs(lon_tens):03d}'
+    filename = f'DEM_{lat_cardinal}{np.abs(lat):02d}_00_{lon_cardinal}{np.abs(lon):03d}_00_C01.tif'
     file_url = f'{URL}/{prefix}/{filename}'
     return file_url
 
diff --git a/src/multirtc/fetch.py b/src/multirtc/fetch.py
index 2f50081..2a5ed43 100644
--- a/src/multirtc/fetch.py
+++ b/src/multirtc/fetch.py
@@ -63,13 +63,25 @@ def download_file(
     session.mount('https://', HTTPAdapter(max_retries=retry_strategy))
     session.mount('http://', HTTPAdapter(max_retries=retry_strategy))
 
-    with session.get(url, stream=True) as s:
-        download_path = _get_download_path(s.url, s.headers.get('content-disposition'), directory)
-        s.raise_for_status()
-        with open(download_path, 'wb') as f:
-            for chunk in s.iter_content(chunk_size=chunk_size):
-                if chunk:
-                    f.write(chunk)
-    session.close()
+    try:
+        with session.get(url, stream=True) as s:
+            download_path = _get_download_path(s.url, s.headers.get('content-disposition'), directory)
+            s.raise_for_status()
+            bytes_written = 0
+            with open(download_path, 'wb') as f:
+                for chunk in s.iter_content(chunk_size=chunk_size):
+                    if chunk:
+                        f.write(chunk)
+                        bytes_written += len(chunk)
+            if bytes_written == 0:
+                msg = f'Downloaded 0 bytes from {url}'
+                logging.error(msg)
+                return ''
+            logging.info(f'Download successful: {url}')
+    except requests.exceptions.RequestException as e:
+        logging.exception('Download failed: {url}, {e}')
+        return ''
+    finally:
+        session.close()
 
     return str(download_path)