diff --git a/.gitignore b/.gitignore
index 199c7c6..f4f44bf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -64,3 +64,4 @@ data
 # PyTest
 .pytest_cache/
 virtualenv
+data/
diff --git a/planetutils/elevation_tile_download.py b/planetutils/elevation_tile_download.py
index 0959087..b7629d4 100644
--- a/planetutils/elevation_tile_download.py
+++ b/planetutils/elevation_tile_download.py
@@ -7,9 +7,11 @@
 from .bbox import load_features_csv, load_feature_string
 from .elevation_tile_downloader import ElevationGeotiffDownloader, ElevationSkadiDownloader
 
+
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument('--outpath', help='Output path for elevation tiles.', default='.')
+    parser.add_argument('--existpath', help='Directory to check for existing files.', default=None)
     parser.add_argument('--csv', help='Path to CSV file with bounding box definitions.')
     parser.add_argument('--bbox', help='Bounding box for extract file. Format for coordinates: left,bottom,right,top')
     parser.add_argument('--verbose', help="Verbose output", action='store_true')
@@ -22,11 +24,11 @@ def main():
         log.set_verbose()
 
     if args.format == 'geotiff':
-        p = ElevationGeotiffDownloader(args.outpath, zoom=args.zoom)
+        p = ElevationGeotiffDownloader(args.outpath, zoom=args.zoom, exist_path=args.existpath)
     elif args.format == 'skadi':
         p = ElevationSkadiDownloader(args.outpath)
     else:
-        print("Unknown format: %s"%args.format)
+        print("Unknown format: %s" % args.format)
         sys.exit(1)
 
     if args.csv:
diff --git a/planetutils/elevation_tile_downloader.py b/planetutils/elevation_tile_downloader.py
index c03613e..c5d6ecb 100644
--- a/planetutils/elevation_tile_downloader.py
+++ b/planetutils/elevation_tile_downloader.py
@@ -1,12 +1,19 @@
 #!/usr/bin/env python
 from __future__ import absolute_import, unicode_literals
+
 import os
-import subprocess
 import math
 
-from . import download
-from . import log
-from .bbox import validate_bbox
+import urllib3
+from retry import retry
+from concurrent import futures
+
+from urllib3 import Timeout
+
+from planetutils import download
+from planetutils import log
+from planetutils.bbox import validate_bbox
+
 
 def makedirs(path):
     try:
@@ -14,9 +21,15 @@ def makedirs(path):
     except OSError as e:
         pass
 
+
 class ElevationDownloader(object):
-    def __init__(self, outpath='.'):
+    zoom = 0
+    timeout = Timeout(connect=3.0, read=7.0)
+    http = urllib3.PoolManager(maxsize=50, timeout=timeout)
+
+    def __init__(self, outpath='.', exist_path=None):
         self.outpath = outpath
+        self.exist_path = exist_path
 
     def download_planet(self):
         self.download_bbox([-180, -90, 180, 90])
@@ -24,36 +37,52 @@ def download_planet(self):
     def download_bboxes(self, bboxes):
         for name, bbox in bboxes.items():
             self.download_bbox(bbox)
-    
-    def download_bbox(self, bbox, bucket='elevation-tiles-prod', prefix='geotiff'):
+
+    def filter_needed(self, bbox):
         tiles = self.get_bbox_tiles(bbox)
         found = set()
         download = set()
-        for z,x,y in tiles:
-            od = self.tile_path(z, x, y)
-            op = os.path.join(self.outpath, *od)
-            if self.tile_exists(op):
-                found.add((x,y))
-            else:
-                download.add((x,y))
-        log.info("found %s tiles; %s to download"%(len(found), len(download)))
-        for x,y in sorted(download):
-            self.download_tile(bucket, prefix, z, x, y)
+        exist_dir = self.exist_path if self.exist_path else self.outpath
+        for root, dirs, files in os.walk(exist_dir):
+            path = root.split(os.sep)
+            for file in files:
+                if '.tif' in file:
+                    found.add("%s/%s/%s" % (path[-2], path[-1], file.split('.')[0]))
+        for z, x, y in tiles:
+            if '%s/%s/%s' % (z, x, y) not in found:
+                download.add((x, y))
+        log.info("found %s tiles; %s to download" % (len(found), len(download)))
+        return download
+
+    def download_bbox(self, bbox, bucket='elevation-tiles-prod', prefix='geotiff'):
+        download = self.filter_needed(bbox)
+        tasks = {self._tile_url_path(bucket, prefix, self.zoom, x, y) for x, y in download}
+
+        with futures.ThreadPoolExecutor() as executor:
+            # Start the load operations and mark each future with its URL
+            future_to_url = {
+                executor.submit(self._download_multi, url_op): url_op for url_op in tasks
+            }
+            for future in futures.as_completed(future_to_url):
+                try:
+                    future.result()
+                except Exception as exc:
+                    log.error('generated an exception: %s' % exc)
+                    pass
 
     def tile_exists(self, op):
         if os.path.exists(op):
             return True
 
-    def download_tile(self, bucket, prefix, z, x, y, suffix=''):
+    def _tile_url_path(self, bucket, prefix, z, x, y, suffix=''):
         od = self.tile_path(z, x, y)
         op = os.path.join(self.outpath, *od)
         makedirs(os.path.join(self.outpath, *od[:-1]))
         if prefix:
             od = [prefix]+od
-        url = 'http://s3.amazonaws.com/%s/%s%s'%(bucket, '/'.join(od), suffix)
-        log.info("downloading %s to %s"%(url, op))
-        self._download(url, op)
-        
+        url = 'http://s3.amazonaws.com/%s/%s%s' % (bucket, '/'.join(od), suffix)
+        return url, op
+
     def tile_path(self, z, x, y):
         raise NotImplementedError
 
@@ -63,6 +92,18 @@ def get_bbox_tiles(self, bbox):
     def _download(self, url, op):
         download.download(url, op)
 
+    @retry(exceptions=Exception, tries=5, delay=2, backoff=2, logger=log)
+    def _download_multi(self, url_op):
+        url, op = url_op
+        log.info("downloading %s to %s" % (url, op))
+        request = self.http.request('GET', url)
+        with open(op, 'wb') as f:
+            try:
+                f.write(request.data)
+            except Exception as exc:
+                raise Exception("Error downloading %r - %s", (url, exc))
+
+
 class ElevationGeotiffDownloader(ElevationDownloader):
     def __init__(self, *args, **kwargs):
         self.zoom = kwargs.pop('zoom', 0)
@@ -80,7 +121,7 @@ def get_bbox_tiles(self, bbox):
         size = 2**self.zoom
         xt = lambda x:int((x + 180.0) / 360.0 * size)
         yt = lambda y:int((1.0 - math.log(math.tan(math.radians(y)) + (1 / math.cos(math.radians(y)))) / math.pi) / 2.0 * size)
-        tiles = []    
+        tiles = []
         for x in range(xt(left), xt(right)+1):
             for y in range(yt(top), yt(bottom)+1):
                 tiles.append([self.zoom, x, y])
@@ -89,9 +130,10 @@ def get_bbox_tiles(self, bbox):
     def tile_path(self, z, x, y):
         return list(map(str, [z, x, str(y)+'.tif']))
 
+
 class ElevationSkadiDownloader(ElevationDownloader):
     HGT_SIZE = (3601 * 3601 * 2)
-    
+
     def get_bbox_tiles(self, bbox):
         left, bottom, right, top = validate_bbox(bbox)
         min_x = int(math.floor(left))
@@ -104,13 +146,13 @@ def get_bbox_tiles(self, bbox):
             for y in range(min_y, max_y):
                 tiles.add((0, x, y))
         return tiles
-    
+
     def tile_exists(self, op):
-        if os.path.exists(op) and os.stat(op).st_size == self.HGT_SIZE:	
+        if os.path.exists(op) and os.stat(op).st_size == self.HGT_SIZE:
             return True
 
     def download_tile(self, bucket, prefix, z, x, y, suffix=''):
-        super(ElevationSkadiDownloader, self).download_tile(bucket, 'skadi', z, x, y, suffix='.gz')
+        super(ElevationSkadiDownloader, self)._tile_url_path(bucket, 'skadi', z, x, y, suffix='.gz')
 
     def tile_path(self, z, x, y):
         ns = lambda i:'S%02d'%abs(i) if i < 0 else 'N%02d'%abs(i)
diff --git a/planetutils/log.py b/planetutils/log.py
index 89e7bde..337471f 100644
--- a/planetutils/log.py
+++ b/planetutils/log.py
@@ -2,18 +2,22 @@
 logging.basicConfig(format='[%(levelname)s] %(message)s')
 logger = logging.getLogger(__name__)
 
+
 def set_quiet():
     logger.setLevel(logging.ERROR)
 
+
 def set_verbose():
     logger.setLevel(logging.DEBUG)
 
+
 def set_default():
     logger.setLevel(logging.INFO)
 
+
 set_default()
 
 info = logger.info
 debug = logger.debug
 warning = logger.warning
-error = logger.error
\ No newline at end of file
+error = logger.error
diff --git a/setup.py b/setup.py
index 1aa90dd..e70a7d3 100644
--- a/setup.py
+++ b/setup.py
@@ -17,7 +17,7 @@
     author_email='ian@interline.io',
     license='MIT',
     packages=find_packages(exclude=['contrib', 'docs', 'tests']),
-    install_requires=['future', 'requests'], #, 'osmium', 'boto3'
+    install_requires=['future', 'requests', 'retry'],  #, 'osmium', 'boto3', 'gdal'
     tests_require=['nose'],
     test_suite = 'nose.collector',
     entry_points={
@@ -28,6 +28,7 @@
             'osm_extract_download=planetutils.osm_extract_download:main',
             'elevation_tile_download=planetutils.elevation_tile_download:main',
             'elevation_tile_merge=planetutils.elevation_tile_merge:main',
+            'elevation_tile_terrain=planetutils.elevation_tile_terrain:main',
             'valhalla_tilepack_download=planetutils.tilepack_download:main',
             'valhalla_tilepack_list=planetutils.tilepack_list:main'
         ],