Troubleshooting Mkgmap Splitter: Common Issues and Fixes

Troubleshooting Mkgmap Splitter: Common Issues and FixesMkgmap splitter is a utility used alongside mkgmap (the tool that converts OpenStreetMap data into Garmin-compatible .img files) to split large OSM files into smaller chunks so mkgmap can process them reliably. While splitter makes handling big datasets practical, users often encounter issues ranging from incorrect splits and missing data to performance bottlenecks. This article covers the most common problems, how to diagnose them, and practical fixes.

1. Understanding how the splitter works (brief)

Mkgmap splitter divides input OSM XML/PBF data into tile-sized or feature-count–based chunks. Splitting is typically geographic (by latitude/longitude ranges or tile grid) or by element count. Knowing which mode you’re using helps diagnose problems: geographic splitting preserves spatial boundaries; element-count splitting keeps each chunk under a set object limit.

2. Common problem: Split files missing nodes/ways/relations

Symptoms:

• Output .osm/.pbf chunks cause mkgmap to report missing nodes for ways or broken relations. Cause:
• When splitting, ways and relations that span chunk boundaries may reference nodes located in other chunks. If the splitter doesn’t include dependent nodes/relations, chunks become internally inconsistent. Fixes:
• Use a splitter mode that includes dependent objects. Many splitters have an option like –complete-ways or –complete-rels to include referenced nodes/ways/relations in the same chunk.
• Post-process with a tool that resolves dependencies (for example Osmium or Osmconvert) using options such as –complete-ways or –add-bbox to ensure referenced nodes are retained.
• If your splitter supports overlap tiles (a small buffer around each tile), enable overlap so objects crossing boundaries are duplicated into adjacent chunks.

Example commands:

• osmconvert –complete-ways input.pbf -o=chunk.osm.pbf
• osmium extract –set-bounds –complete-ways

3. Common problem: Excessive duplication of features across chunks

Symptoms:

• Resulting Garmin map includes duplicate features or inflated size because objects were duplicated into overlapping tiles. Cause:
• Overlap buffers or “complete-ways” options duplicate objects so each tile is self-contained. This is expected behavior but can be excessive if overlap size is too large or if duplicated features aren’t pruned before building maps. Fixes:
• Reduce overlap size to the minimum required to keep ways intact across boundaries (e.g., a few meters or a small tile margin).
• After splitting, deduplicate identical objects using tools like Osmosis or Osmconvert where possible; some workflows remove duplicates before passing chunks to mkgmap.
• Use mkgmap’s –family-id and –product-id consistently so duplicates across tiles merge correctly in the final .img (mkgmap can merge objects during compilation if configured properly).

4. Common problem: Splitter crashes or runs out of memory

Symptoms:

• Process terminates with “OutOfMemoryError” or simply stalls. Cause:
• Large datasets, insufficient heap for Java-based tools, or limited system RAM. Fixes:
• Increase Java heap size for Java-based splitters (e.g., java -Xmx4G -jar splitter.jar …). Start with -Xmx set to half-to-two-thirds of available RAM.
• Run splitting on a machine with more RAM or use streaming tools (osmium/osmconvert) that use lower memory footprints.
• Split the dataset in coarse steps (first by large regions, then further split those regions) to reduce peak memory usage.
• Ensure temporary directories have enough space; set JAVA_TMP or the tool’s temp dir option to a drive with ample free space.

5. Common problem: Incorrect tile boundaries / misaligned tiles

Symptoms:

• Tiles do not align with expected grid; neighboring tiles overlap incorrectly or have gaps. Cause:
• Mismatch between coordinate systems, rounding errors, or incorrect tile-size parameters. Fixes:
• Confirm the splitter and mkgmap use the same tiling scheme (e.g., fixed-degree tiles vs. Mercator tiles). Use explicit tile-size parameters if available.
• Use integer-based tile grid parameters where possible to avoid floating-point rounding issues.
• If splitting by tile index, verify your tile-generation math. For example, latitude/longitude ranges should be computed consistently and inclusive/exclusive endpoints handled deliberately.

6. Common problem: Slow splitting performance

Symptoms:

• Splitting takes hours or days for large datasets. Cause:
• Inefficient I/O, single-threaded processing, or unnecessary operations (like XML parsing when PBF could be used). Fixes:
• Prefer PBF input/output over XML (smaller, faster to parse).
• Use tools optimized for speed (osmium, osmconvert) rather than slower XML-based tools.
• Enable multi-threading if the tool supports it.
• Ensure the storage medium is fast (SSD vs. HDD) and not saturated.
• If possible, limit the dataset to the needed bounding box before splitting.

Practical tweak:

• Convert input to PBF, then split: osmium convert -f pbf -o input.pbf input.osm; osmium extract -p polygon.geojson -o chunk.pbf input.pbf

7. Common problem: Charset/encoding issues in tags or names

Symptoms:

• Names or tags show as garbled characters in mkgmap output. Cause:
• Incorrect file encoding or mishandling of UTF-8 when reading/writing OSM data. Fixes:
• Ensure all tools read/write UTF-8. Most modern OSM tools expect UTF-8; avoid conversions to other encodings.
• When using intermediate scripts, explicitly open files with UTF-8 encoding (for example, in Python: open(filename, encoding=“utf-8”)).
• Validate the original OSM file with osmium or other validators to ensure correct UTF-8 bytes.

8. Common problem: mkgmap