diff --git a/.github/spell-ignore-words.txt b/.github/spell-ignore-words.txt
index cecdf78c2..511234cba 100644
--- a/.github/spell-ignore-words.txt
+++ b/.github/spell-ignore-words.txt
@@ -10,3 +10,4 @@ Mapnik
 Redis
 CloudFront
 OpenLayers
+pyproj
diff --git a/poetry.lock b/poetry.lock
index 2909cad59..fe8fe9b4f 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -3490,4 +3490,4 @@ test = ["zope.testing"]
 [metadata]
 lock-version = "2.0"
 python-versions = ">=3.10,<3.13"
-content-hash = "977c579434bd4b21a46e5bedbbbb39c9fa371f670b88821aaec7fc6049a3861f"
+content-hash = "714596af26b818b77392fc5565e4a7dc5f93927aeea6d1a130bb7ecb98ee1226"
diff --git a/pyproject.toml b/pyproject.toml
index 3d4ed22f8..15a8fede1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -74,6 +74,7 @@ waitress = "3.0.2"
 certifi = "2024.8.30"
 Paste = "3.10.1"
 psutil = "6.1.0"
+pyproj = "3.7.0"
 
 [tool.poetry.group.dev.dependencies]
 prospector = { extras = ["with_mypy", "with_bandit", "with_pyroma", "with_ruff"], version = "1.13.3" }
diff --git a/tilecloud_chain/CONFIG.md b/tilecloud_chain/CONFIG.md
index 45fd88a9d..927aacee3 100644
--- a/tilecloud_chain/CONFIG.md
+++ b/tilecloud_chain/CONFIG.md
@@ -23,7 +23,7 @@
 - **`postgresql`**: Refer to _[#/definitions/postgresql](#definitions/postgresql)_.
 - **`openlayers`** _(object)_: Configuration used to generate the OpenLayers example page. Cannot contain additional properties.
   - **`srs`** _(string)_: The projection code. Default: `"EPSG:2056"`.
-  - **`proj4js_def`** _(string)_: The `proj4js` definition, get it from https://epsg.io/. Default: `"+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=2600000 +y_0=1200000 +ellps=bessel +towgs84=674.374,15.056,405.346,0,0,0,0 +units=m +no_defs"`.
+  - **`proj4js_def`** _(string)_: The `proj4js` definition, by default it will be build with pyproj.
   - **`center_x`** _(number)_: The center easting. Default: `2600000`.
   - **`center_y`** _(number)_: The center northing. Default: `1200000`.
   - **`zoom`** _(number)_: The initial zoom. Default: `3`.
diff --git a/tilecloud_chain/configuration.py b/tilecloud_chain/configuration.py
index abb70a555..4351148f1 100644
--- a/tilecloud_chain/configuration.py
+++ b/tilecloud_chain/configuration.py
@@ -1979,9 +1979,7 @@ class Openlayers(TypedDict, total=False):
     """
     Proj4js definition.
 
-    The `proj4js` definition, get it from https://epsg.io/
-
-    default: +proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=2600000 +y_0=1200000 +ellps=bessel +towgs84=674.374,15.056,405.346,0,0,0,0 +units=m +no_defs
+    The `proj4js` definition, by default it will be build with pyproj
     """
 
     center_x: int | float
@@ -2050,10 +2048,6 @@ class Openlayers(TypedDict, total=False):
 """ Default value of the field path 'Redis prefix' """
 
 
-PROJ4JS_DEFINITION_DEFAULT = "+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=2600000 +y_0=1200000 +ellps=bessel +towgs84=674.374,15.056,405.346,0,0,0,0 +units=m +no_defs"
-""" Default value of the field path 'OpenLayers proj4js_def' """
-
-
 class Phone(TypedDict, total=False):
     """
     Phone.
diff --git a/tilecloud_chain/schema.json b/tilecloud_chain/schema.json
index 5154ef6ad..9f45dd5d6 100644
--- a/tilecloud_chain/schema.json
+++ b/tilecloud_chain/schema.json
@@ -1541,9 +1541,8 @@
         },
         "proj4js_def": {
           "title": "Proj4js definition",
-          "description": "The `proj4js` definition, get it from https://epsg.io/",
-          "type": "string",
-          "default": "+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=2600000 +y_0=1200000 +ellps=bessel +towgs84=674.374,15.056,405.346,0,0,0,0 +units=m +no_defs"
+          "description": "The `proj4js` definition, by default it will be build with pyproj",
+          "type": "string"
         },
         "center_x": {
           "title": "Center x",
diff --git a/tilecloud_chain/views/admin.py b/tilecloud_chain/views/admin.py
index 3fbfbc965..34ebc795b 100644
--- a/tilecloud_chain/views/admin.py
+++ b/tilecloud_chain/views/admin.py
@@ -31,13 +31,13 @@
 import logging
 import multiprocessing
 import os
-import re
 import shlex
 import subprocess  # nosec
 from collections.abc import Callable
 from typing import IO, Any
 from urllib.parse import urljoin
 
+import pyproj
 import pyramid.httpexceptions
 import pyramid.request
 import pyramid.response
@@ -307,15 +307,13 @@ def admin_test(self) -> dict[str, Any]:
         assert self.gene
         config = self.gene.get_host_config(self.request.host)
         main_config = self.gene.get_main_config()
+        srs = config.config["openlayers"].get("srs", configuration.SRS_DEFAULT)
+        proj4js_def = config.config["openlayers"].get("proj4js_def")
+        if proj4js_def is None:
+            proj4js_def = pyproj.CRS.from_string(srs).to_proj4()
         return {
-            "proj4js_def": re.sub(
-                r"\s+",
-                " ",
-                config.config["openlayers"]
-                .get("proj4js_def", configuration.PROJ4JS_DEFINITION_DEFAULT)
-                .strip(),
-            ),
-            "srs": config.config["openlayers"].get("srs", configuration.SRS_DEFAULT),
+            "proj4js_def": proj4js_def,
+            "srs": srs,
             "center_x": config.config["openlayers"].get("center_x", configuration.CENTER_X_DEFAULT),
             "center_y": config.config["openlayers"].get("center_y", configuration.CENTER_Y_DEFAULT),
             "zoom": config.config["openlayers"].get("zoom", configuration.MAP_INITIAL_ZOOM_DEFAULT),