B41 to B42 Map Converter

Small wrapper around Project Zomboid's own zombie.pot.POT converter.

It converts a Build 41 map folder into the Build 42 cell/chunk layout by using the projectzomboid.jar and jre64 already installed with the Steam version of Project Zomboid.

Minimum requirements

• Windows.
• Project Zomboid installed through Steam.
• A Project Zomboid install folder containing:
  • projectzomboid.jar
  • jre64\bin\java.exe
• A Build 41 map folder containing map files such as:
  • *.lotheader
  • world_*.lotpack
  • chunkdata_*.bin

The release zip does not include Project Zomboid files. It uses the local game installation on the user's machine.

Included files

• run.bat - launcher script.
• POTConverterWrapper.jar - tiny Java wrapper that calls zombie.pot.POT.
• src\Main.java - source code for the wrapper.
• build.bat - maintainer helper to rebuild POTConverterWrapper.jar.
• B41\ - default input folder.
• B42\ - default output folder.

Quick use

1. Extract the zip anywhere.
2. Copy only the required Build 41 binary map files into the B41 folder.
3. Double-click run.bat.
4. Converted Build 42 files are written to B42.

By default, run.bat expects Project Zomboid here:

C:\Program Files (x86)\Steam\steamapps\common\ProjectZomboid

Recommended input cleanup

Use a clean input folder. Before running the converter, remove files that are not part of the binary map data being converted.

Recommended files to keep in B41:

• *.lotheader
• world_*.lotpack
• chunkdata_*.bin
• worldmap.xml.bin, if your map has one
• worldmap-forest.xml.bin, if your map has one

Recommended files to leave out:

• objects.lua
• spawnpoints.lua
• worldmap.xml
• biome-map or biomemap files
• vegetation maps, zoning files, editor metadata, mod metadata
• unrelated folders or helper files

Extra files are not converted by this wrapper, and some unexpected files or folders can make the underlying Project Zomboid converter report errors. The safest workflow is to copy only the binary B41 map files listed above into a temporary B41 input folder.

Custom Project Zomboid install path

If Project Zomboid is installed elsewhere, set PZ_HOME before running:

set "PZ_HOME=D:\SteamLibrary\steamapps\common\ProjectZomboid"
run.bat

Command-line use

You can pass input and output folders explicitly:

run.bat "D:\Maps\MyMap_B41" "D:\Maps\MyMap_B42"

This mode is useful for scripts and testing because the window does not pause at the end.

Rebuilding the wrapper

This is only needed if you modify src\Main.java.

Requirements:

• A JDK installed and available in PATH.
• javac and jar commands available.
• A local Project Zomboid install containing projectzomboid.jar.

The Steam jre64 folder is enough to run the tool, but it is only a runtime. It does not include javac.

For the current Steam Build 42 jar, use JDK 25 or newer. If javac prints an error like class file has wrong version 69.0, the JDK is too old for the current projectzomboid.jar.

Automatic rebuild:

build.bat

If Project Zomboid is installed elsewhere:

set "PZ_HOME=D:\SteamLibrary\steamapps\common\ProjectZomboid"
build.bat

Manual rebuild commands:

mkdir build\classes
javac -encoding UTF-8 -cp "%PZ_HOME%\projectzomboid.jar" -d build\classes src\Main.java
jar --create --file POTConverterWrapper.jar -C build\classes Main.class

After rebuilding, run a small conversion test before publishing a release zip.

Scope

This tool is only a binary Build 41 -> Build 42 map-data converter.

It is not a full map/mod migration tool.

What it converts

The wrapper calls:

new zombie.pot.POT().convertMapDirectory(input, output)

In the current Project Zomboid Build 42 code path, this covers the map binary cell data handled by POT, notably lot headers, lot packs and chunkdata. If worldmap.xml.bin or worldmap-forest.xml.bin exist, the game converter also attempts to convert those binary world-map files.

Plain Lua/XML helper files such as objects.lua, spawnpoints.lua and worldmap.xml are not transformed by this wrapper.

What it does not do

• It does not necessarily update or rewrite objects.lua.
• It does not update or rewrite spawnpoints.lua.
• It does not generate or migrate biome-map data, biomemap files, vegetation maps, zoning data, or other editor-side metadata.
• It does not rebuild a full Project Zomboid map project.
• It does not convert custom scripts, tilesets, textures, loot tables, map descriptions, mod metadata, or Workshop packaging.
• It does not validate that the converted map is gameplay-complete in Build 42.

In practical terms, treat the output as converted binary map cells, not as a complete Build 42-ready mod package.

Troubleshooting

Missing Java runtime

The script could not find:

%PZ_HOME%\jre64\bin\java.exe

Set PZ_HOME to the real Project Zomboid install folder.

Missing Project Zomboid jar

The script could not find:

%PZ_HOME%\projectzomboid.jar

Set PZ_HOME to the real Project Zomboid install folder.

No .lotheader files found

The input folder is not a Build 41 map data folder, or the map files were copied to the wrong place. Put the source map files in B41, or pass the source folder as the first command-line argument.

Notes for GitHub releases

Recommended release asset:

B41toB42-Converter-v1.0.0.zip

Do not include projectzomboid.jar, jre64, or any other Project Zomboid game files in the release asset. The user must supply those through their own local Project Zomboid installation.

Build notes

The included wrapper class was verified by running it against the Steam installation's projectzomboid.jar and jre64.

The included ready-to-use jar may still run even if the machine has no JDK, because run.bat uses Project Zomboid's bundled jre64 runtime.