diff --git a/app/apps/api/serializers.py b/app/apps/api/serializers.py
index e69add839008d6dc165a8575f89a6b53b472510a..081a3cf0c5b6919bc2282b334289f17d6dbb57df 100644
--- a/app/apps/api/serializers.py
+++ b/app/apps/api/serializers.py
@@ -133,7 +133,8 @@ class PartSerializer(serializers.ModelSerializer):
             'workflow',
             'order',
             'recoverable',
-            'transcription_progress'
+            'transcription_progress',
+            'source'
         )
 
     def create(self, data):
diff --git a/app/escriptorium/templates/export/alto.xml b/app/escriptorium/templates/export/alto.xml
index ed28bb0e81b8cc6b01fdc868ec76cd11740c2e7e..80b0afbab80e820df539b3e032c8a7bbf8b6e665 100644
--- a/app/escriptorium/templates/export/alto.xml
+++ b/app/escriptorium/templates/export/alto.xml
@@ -1,11 +1,12 @@
 {% load export_tags %}<?xml version="1.0" encoding="UTF-8"?>
 <alto xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns="http://www.loc.gov/standards/alto/ns-v4#"
-      xsi:schemaLocation="http://www.loc.gov/standards/alto/ns-v4# https://gitlab.inria.fr/scripta/escriptorium/-/raw/develop/app/escriptorium/static/alto-4-1-baselines.xsd">
+      xsi:schemaLocation="http://www.loc.gov/standards/alto/ns-v4# http://www.loc.gov/standards/alto/v4/alto-4-2.xsd">
   <Description>
     <MeasurementUnit>pixel</MeasurementUnit>
     <sourceImageInformation>
       <fileName>{{ part.filename }}</fileName>
+      {% if part.source %}<fileIdentifier>{{ part.source }}</fileIdentifier>{% endif %}
     </sourceImageInformation>
   </Description>
   {% if valid_block_types or valid_line_types %}
diff --git a/app/escriptorium/templates/export/pagexml.xml b/app/escriptorium/templates/export/pagexml.xml
index f0be96718103b82a48bed29cefaaa2b8f6f4cdb9..73ded4ccf75b09cd5f0d11aecafaf9dffd502ed8 100644
--- a/app/escriptorium/templates/export/pagexml.xml
+++ b/app/escriptorium/templates/export/pagexml.xml
@@ -1,9 +1,10 @@
 {% load export_tags %}<?xml version="1.0" encoding="UTF-8"  standalone="yes"?>
 <PcGts xmlns="http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15 http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15/pagecontent.xsd">
-  <Metadata>
+  <Metadata{% if part.source %} externalRef="{{part.source}}"{% endif %}>
 	<Creator>escriptorium</Creator>
 	<Created>{% current_time %}</Created>
-    <LastChange>{% current_time %}</LastChange>
+        <LastChange>{% current_time %}</LastChange>
+
   </Metadata>
   <Page imageFilename="{{ part.filename }}" imageWidth="{{ part.image.width  }}" imageHeight="{{ part.image.height  }}">