Automating Map Merge in OziExplorer: Workflow & ScriptsMerging multiple raster maps into a single seamless map for use in OziExplorer can be time-consuming when done by hand. Automating the process saves time, reduces human error, and makes repeated map-building tasks reproducible. This article explains an end-to-end automated workflow for merging maps into OziExplorer format, covers common pitfalls, and provides example scripts (Windows and cross‑platform) you can adapt.
Overview and goals
Automating map merge for OziExplorer usually aims to:
- Combine multiple raster tiles (JPEG/TIFF/PNG) into one large georeferenced image.
- Preserve or reconstruct georeferencing so OziExplorer can use the resulting image with waypoints and tracks.
- Split or reproject tiles when needed to match a target projection and pixel resolution.
- Produce an OziExplorer map file (.map) and optionally associated calibration points if automatic georeferencing isn’t embedded.
Key constraints:
- OziExplorer primarily uses simple georeferencing (pixel coordinates to lat/long or grid). It supports both WGS84 lat/long and various projected grids via control points or world files.
- Raster formats: TIFF (GeoTIFF) with embedded georeference is easiest for automation; JPEG/PNG require separate world files or conversion.
- Accuracy is limited by source georeferencing and how well tiles align.
Tools you’ll need
- GDAL (Geospatial Data Abstraction Library) — core tool for reprojection, warping, mosaicking, and exporting world files.
- ImageMagick (optional) — for image format conversions and processing.
- A scripting language — PowerShell or batch for Windows; Bash/Python for cross‑platform automation.
- OziExplorer (for final testing and any manual calibration).
- Optional: maptrim/MapMerge utilities or custom utilities if you already have them.
Install GDAL (includes gdalwarp, gdal_merge.py, gdal_translate). On Windows use OSGeo4W or standalone binaries; on Linux/macOS use package managers (apt, brew).
Workflow steps (high level)
- Inventory and inspect source tiles (formats, CRS, resolution).
- Normalize CRS and pixel resolution (reproject/warp to target CRS).
- Align and mosaic tiles into a single image.
- Convert mosaic into an OziExplorer‑friendly image format and create the .map file or world file and calibration points.
- Validate in OziExplorer and adjust control points if necessary.
- Optional: split the final map if it exceeds practical size limits for OziExplorer.
Detailed step-by-step process
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Inventory and inspect
- Use gdalinfo to list CRS, bounds, pixel size:
gdalinfo tile1.tif - Record projection (EPSG code), pixel size, and any rotation/skew.
- Use gdalinfo to list CRS, bounds, pixel size:
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Choose a target CRS and resolution
- For OziExplorer use WGS84 geographic (EPSG:4326) if you want simple lat/long calibration. If your workflow or local grid requires a projected CRS (e.g., UTM), pick the appropriate EPSG.
- Decide a target pixel size (degrees per pixel for EPSG:4326 or meters per pixel for projected).
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Reproject/warp tiles (if needed)
- Reproject each tile to the target CRS and resolution with gdalwarp:
gdalwarp -t_srs EPSG:4326 -tr 0.0001 0.0001 -r bilinear input.tif warped.tif- -tr sets target resolution (x y). Use a value that matches or is a common multiple of source resolutions.
- -r sets resampling (near, bilinear, cubic).
- Reproject each tile to the target CRS and resolution with gdalwarp:
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Mosaic/merge tiles
- Use gdalwarp to mosaic multiple inputs in one step (handles reprojection, nodata, and merging):
gdalwarp -t_srs EPSG:4326 -tr 0.0001 0.0001 -r bilinear tile1.tif tile2.tif tile3.tif merged.tif - Or use gdal_merge.py (for same CRS inputs):
gdal_merge.py -o merged.tif -of GTiff tile1.tif tile2.tif tile3.tif
- Use gdalwarp to mosaic multiple inputs in one step (handles reprojection, nodata, and merging):
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Create an image format and georeference for OziExplorer
- OziExplorer can use an image plus a .map file. The .map file contains calibration control points and projection info.
- Option A — Generate a simple world file + projection:
- Export GeoTIFF to a TIFF/JPG and write a world file (.wld/.jgw) using gdal_translate:
gdal_translate -of JPEG merged.tif merged.jpg gdal_translate -of GTiff merged.tif merged_geo.tif - Use gdalinfo on merged.tif to get geotransform values and create a .wld manually or use gdal_translate options to produce world file (some versions support -co WORLDFILE=YES).
- Export GeoTIFF to a TIFF/JPG and write a world file (.wld/.jgw) using gdal_translate:
- Option B — Create an OziExplorer .map file automatically:
- The .map format is text-based and needs at least 4 corner calibration points or a set of ground control points plus projection header. You can extract corner coordinates from the merged GeoTIFF and write a .map file with those corners.
- Extract bounds and geotransform:
gdalinfo merged.tifNote Origin (top-left) and Pixel Size; compute four corners:
- Top-left (Xmin, Ymax), Top-right (Xmax, Ymax), Bottom-right (Xmax, Ymin), Bottom-left (Xmin, Ymin). - Convert projected coords to lat/long if needed (use gdalwarp or ogr2ogr/pyproj).
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Validate and refine in OziExplorer
- Load the image and .map file; check alignment with known waypoints or GPS track. Fine-tune control points in OziExplorer if small adjustments are needed.
Example .map file generator (Python)
Below is a concise Python script that reads a GeoTIFF, extracts corner lat/longs, and writes a minimal OziExplorer .map file. Requires GDAL Python bindings or rasterio + pyproj.
#!/usr/bin/env python3 import sys from osgeo import gdal, osr if len(sys.argv) < 3: print("Usage: make_ozi_map.py merged.tif output.map") sys.exit(1) src = gdal.Open(sys.argv[1]) gt = src.GetGeoTransform() cols = src.RasterXSize rows = src.RasterYSize def pixel_to_map(px, py): x = gt[0] + px*gt[1] + py*gt[2] y = gt[3] + px*gt[4] + py*gt[5] return x, y # corners in source CRS corners = [ pixel_to_map(0, 0), # top-left pixel_to_map(cols, 0), # top-right pixel_to_map(cols, rows), # bottom-right pixel_to_map(0, rows), # bottom-left ] # transform to WGS84 if needed src_srs = osr.SpatialReference() src_srs.ImportFromWkt(src.GetProjection()) tgt_srs = osr.SpatialReference() tgt_srs.ImportFromEPSG(4326) ct = osr.CoordinateTransformation(src_srs, tgt_srs) llcorners = [ct.TransformPoint(x, y) for (x, y) in corners] # write minimal .map out = open(sys.argv[2], "w") out.write("OziExplorer Map Data File Version 2.2 ") out.write("merged.jpg ") # image file name expected out.write("1 ,Map Generated by Script ") out.write("WGS 84 ") out.write("Reserved 0 ") out.write("Magnetic Variation,,,,,,,E ") out.write("0.0000,0.0000,0.0000,WGS 84 ") out.write(" ") # write 4 calibration points for i, (lon, lat, z) in enumerate(llcorners, start=1): # pixel coords if i == 1: px, py = 0, 0 elif i == 2: px, py = cols, 0 elif i == 3: px, py = cols, rows else: px, py = 0, rows out.write(f"{i}, ,Map Code, ,{lat:.8f},{lon:.8f},,{px},{py}, ") out.close() print("Wrote", sys.argv[2]) Notes:
- The script assumes an accompanying merged.jpg (you should convert merged.tif to JPEG or adjust filename).
- OziExplorer expects lat,lon order in its .map entries for geographic maps.
- Check file format specifics against your OziExplorer version; tweak header lines as needed.
Example Windows batch + GDAL commands
- Reproject and resample each tile:
for %%f in (*.tif) do gdalwarp -t_srs EPSG:4326 -tr 0.0001 0.0001 -r bilinear "%%f" "warped%%~nf_warp.tif" - Merge warped tiles:
gdalwarp -t_srs EPSG:4326 -tr 0.0001 0.0001 -r bilinear warped*_warp.tif merged.tif - Create JPEG and world file:
gdal_translate -of JPEG merged.tif merged.jpg gdal_translate -of GTiff -co WORLDFILE=YES merged.tif merged_world.tif - Run the Python .map generator above to produce merged.map.
Handling common issues
- Misalignment between tiles: ensure consistent CRS and pixel resolution before merging. Use ground control points if source georeferencing is poor.
- Color/contrast seam lines: use histogram matching or feathering with gdalwarp -wo SOURCE_EXTRA or use blending options in more advanced tools.
- Large files and memory: create tiled, compressed GeoTIFFs (-co TILED=YES -co COMPRESS=DEFLATE) or build overviews.
- Rotation/skewed images: gdalwarp handles affine transforms, but ensure source geotransforms are applied properly.
Tips and best practices
- Work in GeoTIFF until final export to preserve georeference.
- Use a consistent naming convention for tiles including bounding box or row/column indices.
- Produce and keep a manifest (CSV) of source tiles with their CRS and pixel sizes; helps debugging later.
- If you’ll do repeated updates, script metadata checks (gdalinfo parsing) so the pipeline can abort cleanly on mismatched inputs.
- Use small test sets before running large mosaics.
When to use manual calibration in OziExplorer
If automatic georeferencing results in small but visible misfits, load the merged image into OziExplorer and add a few accurate control points (tie to known GPS waypoints or map junctions). Manual control points can correct local distortions that global reprojection/tools can’t fix.
Conclusion
Automating map merge for OziExplorer relies on GDAL for accurate reprojection and mosaicking, simple scripting to glue steps together, and a small map-file generator to produce OziExplorer-compatible calibration. Start by standardizing CRS and resolution, keep your workflow reproducible, and validate the final map in OziExplorer, adding manual control points only when needed.
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