diff --git a/topics/data_intro.html b/topics/data_intro.html
index 4e660bb6..85de5e59 100644
--- a/topics/data_intro.html
+++ b/topics/data_intro.html
@@ -18,20 +18,18 @@ <h3>Lecture</h3>
     <li>Coordinate referenced systems 
     <ul>
     <!--<li><a href="https://thetruesize.com/">The true size</a> - explore the size of countries and their distortions in Pseudo Mercator-->
-    <!--<li><a href="https://www.gaia-gis.it/fossil/libspatialite/wiki?name=PROJ.6">Transition to PROJ.6 explained</a>-->
     <!-- Notes on <a href="https://fatra.cnr.ncsu.edu/geospatial-modeling-course/resources/WGS84_NAD83.pdf">WGS84 and NAD83 difference</a>-->
-    <li><a href="https://epsg.io/">Coordinate systems and EPSG codes</a>,
-    <li><a href="https://confluence.qps.nl/pages/viewpage.action?pageId=29855153">North American Datums explained</a>,
+    <li><a href="https://epsg.io/">Coordinate systems and EPSG codes</a>, also available at 
+        <a href="https://spatialreference.org/">Spatial Reference</a>
+    <li><a href="https://confluence.qps.nl/pages/viewpage.action?pageId=29855153">North American Datums explained</a>
     <li><a href="https://geodesy.noaa.gov/datums/newdatums/index.shtml">New NA datum in 2022</a>
     <li><a href="https://web.archive.org/web/20160122040117/progonos.com/furuti/mapproj/normal/toc/carttoc.html">Map projections</a>,
     </ul>
     <li>Open source geospatial software
     <ul>
-      <li><a href="https://wenzeslaus.github.io/presentations/geoforum2014-foss.pdf">How Open Source Geospatial Development Works</a>
-      <!--    (<a href="https://cnr.online.ncsu.edu/online/Play/84b7f46cff9c45afa812539109630cfd1d?catalog=b73459f2-e0a5-44fd-b940-ed9e529b2334">recording</a>
-          from Geospatial Forum available at <a href="https://cnr.online.ncsu.edu/online/Catalog/catalogs/geospatial-forum">NCSU</a>)-->
-    <li><a href="https://gistbok.ucgis.org/bok-topics/open-source-software-development">Petras, V., Mitasova, H., and Petrasova, A. (2021). Open Source Software Development. The Geographic Information Science & Technology Body of Knowledge (2nd Quarter 2021 Edition), John P. Wilson (ed.). DOI: 10.22224/gistbok/2021.2.4
+    <li><a href="https://gistbok-ltb.ucgis.org/page/20/concept/5798">Petras, V., Mitasova, H., and Petrasova, A. (2021). Open Source Software Development. The Geographic Information Science & Technology Body of Knowledge (2nd Quarter 2021 Edition), John P. Wilson (ed.). DOI: 10.22224/gistbok/2021.2.4
 </a>
+      <li><a href="https://wenzeslaus.github.io/presentations/geoforum2014-foss.pdf">How Open Source Geospatial Development Works</a>
     </ul>
     <li>Original lecture material: <a href="http://fatra.cnr.ncsu.edu/geospatial-modeling-course/slides/GIS_Anal_Ldata.ppt">Lecture Slides </a> and videos: <a href="https://youtu.be/lgAFtw7cIh4">Mapping</a> and <a href="https://youtu.be/jXFncBsurwo">Georeferencing</a>
 </ul>