diff --git a/README.md b/README.md
index be329d7..5587c1f 100644
--- a/README.md
+++ b/README.md
@@ -1,13 +1,14 @@
 # grassloper
-an application to estimate the slope of a granular surface flow
+GRAnular Surface SLOPER: an application to estimate the slope of a granular surface flow
 
+`grassloper` uses the output of [tractrac](https://perso.univ-rennes1.fr/joris.heyman/trac.html).
 
 # how to install
 
 `grassloper` is delivered as a simple pip application, and as such it can be installed from scratch using the following steps:
 
 ```sh
-# create a python virtual environment (we name it name grassloper.venv)
+# create a python virtual environment (we name it grassloper.venv)
 bob@stykades:~/work/grassloper$ python3 -m venv grassloper.venv  
 # activate the virtual environment to use it
 bob@stykades:~/work/grassloper$ source ./grassloper.venv/bin/activate
diff --git a/grassloper/main.py b/grassloper/main.py
index d6180b8..3d1f689 100755
--- a/grassloper/main.py
+++ b/grassloper/main.py
@@ -1,4 +1,3 @@
-#!/usr/bin/env python3
 import numpy as np
 import cv2
 from pathlib import Path
@@ -200,7 +199,7 @@ def main():
     parser.add_argument('--part-pos-file', type=Path, default=None, required=True, help='the input file containing the particle positions')
     parser.add_argument('--results-file', type=Path, default=None, required=True, help='the output file containing for each frame one line fitting the granular surface (hdf5 file format)')
     parser.add_argument('--part-pos-file-format', type=str, choices=['tractrac-hdf5'], default='tractrac-hdf5', help='the file format of the input file that contains particle positions')
-    parser.add_argument('--particle-radius', type=float, required=True, help='the radius of the particles in pixels', default=11)
+    parser.add_argument('--particle-radius', type=float, required=True, help='the radius of the particles in pixels (make sure that the radius is big enough to unsure that neighbouring particles touch each other)', default=11)
     parser.add_argument('--debug-dir', type=Path, default=None, help='where to store image processing debug files')
     args = parser.parse_args()
     # python3 ./tractrac.git/Python/tractrac.py -f ./grassloper.git/samples/sample002.avi --output 1 -a --saveplot