dwg-extraction

name dwg-extraction description Extract construction intelligence from AutoCAD DWG files by compiling libredwg from source, converting DWG to DXF, and parsing all entity types (survey points, utility structures, contours, property boundaries, construction keynotes, grading data). Use this skill whenever the user uploads or references a .dwg file, mentions AutoCAD drawings, Civil 3D files, grading plans, site plans in DWG format, or wants to extract data from CAD files. Also triggers when processing project documents that include .dwg files, or when the user says "read the DWG", "extract from CAD", "parse the drawing file", "process the civil plans", "what's in the DWG", "survey points from the drawing", "utility data from CAD". This skill handles the entire pipeline from binary DWG to structured JSON data. version 1.0.0 DWG File Extraction Extract structured construction intelligence from AutoCAD DWG files (including Civil 3D). DWG is a proprietary binary format that requires specialized tooling — this skill handles the full pipeline from raw .dwg to structured project data. Pipeline Overview The extraction happens in four stages: Compile libredwg — Build the open-source DWG reader from source (cached after first run) Convert DWG → DXF — Use the compiled dwg2dxf binary to produce a text-based DXF file Parse DXF entities — Custom Python parser extracts all entity types with full attribute data Structure & store — Organize extracted data into plans-spatial.json sections The reason we compile from source rather than using a package manager is that libredwg is not available as an apt package in most environments, and Python DXF libraries (like ezdxf) often fail on Civil 3D DXF output due to malformed MATERIAL entries from proprietary objects. The manual parser in this skill handles these edge cases gracefully. Stage 1: Compile libredwg Check if the binary already exists first. If not, compile it. # Check for cached binary if [ -f /tmp/libredwg/dwg2dxf ]; then echo "dwg2dxf binary found, skipping compilation" else # Clone and compile cd /tmp git clone --depth 1 https://github.com/LibreDWG/libredwg.git cd libredwg # Write the config.h that works (this was developed through trial and error # to handle missing autotools, cmake, and various header conflicts) cat > src/config.h << 'CONFIGEOF' #define PACKAGE "libredwg" #define PACKAGE_VERSION "0.13" #define PACKAGE_STRING "libredwg 0.13" #define LIBREDWG_SO_VERSION "0:13:0" #define HAVE_STDINT_H 1 #define HAVE_STDLIB_H 1 #define HAVE_STRING_H 1 #define HAVE_STRINGS_H 1 #define HAVE_INTTYPES_H 1 #define HAVE_UNISTD_H 1 #define HAVE_CTYPE_H 1 #define HAVE_WCHAR_H 1 #define HAVE_WCTYPE_H 1 #define HAVE_MEMCHR 1 #define HAVE_MEMMEM 1 #define HAVE_SCANDIR 1 #define IS_RELEASE 1 #define PACKAGE_NAME "libredwg" #define HAVE_SYS_STAT_H 1 CONFIGEOF # Compile library object files # Skip in_json.c (needs jsmn.h submodule) and out_geojson.c (additional errors) # These are not needed for DWG→DXF conversion gcc -c -I. -Iinclude -Isrc -DHAVE_CONFIG_H \ src/bits.c src/common.c src/classes.c src/codepages.c \ src/decode.c src/decode_r11.c src/decode_r2007.c \ src/dwg.c src/dwg_api.c src/hash.c src/dynapi.c \ src/dxfclasses.c src/out_dxf.c src/out_json.c \ src/print.c src/free.c src/encode.c -w # Compile the dwg2dxf program gcc -c -I. -Iinclude -Isrc -DHAVE_CONFIG_H programs/dwg2dxf.c -w # Link everything together gcc -o dwg2dxf *.o -lm -w echo "Compilation complete: $(ls -la dwg2dxf) " fi Troubleshooting : If the libredwg repo structure has changed, the key files needed are: src/*.c — Core library (bits, common, classes, codepages, decode, decode_r11, decode_r2007, dwg, dwg_api, hash, dynapi, dxfclasses, out_dxf, out_json, print, free, encode) programs/dwg2dxf.c — The conversion program include/ — Header files src/config.h — Must be created manually (see above) Stage 2: Convert DWG → DXF /tmp/libredwg/dwg2dxf -o /tmp/output.dxf "/path/to/input.dwg" Expect warnings about unhandled Civil 3D objects (AcDbSurface, AcDbAlignment, etc.) — these are normal and don't affect the extraction of standard entities. The conversion produces a text-based DXF that can be parsed line by line. Check the output file exists and has reasonable size (should be several MB for a typical civil site plan). Stage 3: Parse DXF Entities Run the bundled parser script. It handles all the entity types commonly found in construction civil drawings: python3 /path/to/dwg-extraction/scripts/parse_dxf.py /tmp/output.dxf /tmp/dxf_extracted.json The parser extracts these entity categories: Entity Types Extracted Entity DXF Type What It Contains Survey points INSERT + ATTRIB + SEQEND Point number, XYZ coordinates, layer classification Text annotations TEXT Single-line labels (elevations, structure IDs, pipe sizes) Multi-line text MTEXT Construction notes, keynotes, legends Polylines LWPOLYLINE Contour lines, property boundaries, utility runs Lines LINE Grid lines, dimensions, leaders Arcs ARC Radius returns, curved features Circles CIRCLE Manholes, cleanouts, utility symbols Points POINT Control points, spot elevations Hatches HATCH Paving areas, building footprints, material fills Block inserts INSERT Symbols (manholes, valves, meters, trees, fixtures) Layer Naming Convention (Civil 3D) Civil 3D uses a hierarchical layer naming system. The parser categorizes entities by layer prefix to organize the extraction: Layer Pattern Category Typical Content C-STRM-* Storm sewer Pipes, structures, inlets, catch basins C-SSWR-* Sanitary sewer Manholes, pipe runs, cleanouts C-WATR-* Water Mains, services, meters, hydrants C-TOPO-* Topography Existing contours, spot elevations C-PROP-* Property Boundaries, bearings, easements C-ROAD-* Roadway Curbs, pavement, striping C-EROS-* Erosion control Silt fence, inlet protection, BMPs C-BLDG-* Building Footprint, FFE, setbacks C-SITE-* Site Grading, paving, landscaping C-SV$* Survey Survey points, control, benchmarks C-ANNO-* Annotation Dimensions, labels, notes V-* Existing (visible) Existing utilities, features 0 / Defpoints System Default layer, definition points Layers not matching these patterns are still extracted and categorized as "other". Survey Point Extraction Detail Survey points in Civil 3D are stored as INSERT entities on survey point layers (typically containing SV or PNT in the layer name). Each INSERT has child ATTRIB entities that carry the point number. The XYZ coordinates come from the INSERT's insertion point (group codes 10/20/30). The parser tracks the INSERT → ATTRIB → SEQEND sequence to properly group attributes with their parent block. This is important because a naive entity-by-entity parser would lose the parent-child relationship. Structure Proximity Grouping Utility structures (manholes, catch basins, junction boxes) have their attributes (rim elevation, invert elevations, pipe sizes) stored as separate TEXT entities positioned near the structure symbol. The parser groups these using proximity analysis: Collect all TEXT entities on utility layers For each text, find all other texts within a configurable radius (default: 15 units in state plane feet) Group nearby texts into structure records Parse elevation patterns (e.g., "RIM=739.90'", "INV=736.62'(12")") This approach handles the fact that CAD drawings don't have a formal "structure object" — they're composed of individual text entities placed visually near each other. Stage 4: Structure & Store After parsing, organize the extracted data into sections for plans-spatial.json: Data Sections Section Key Content Source Entities dwg_survey_data All survey points with XYZ, elevation stats, control points INSERT+ATTRIB on SV/PNT layers dwg_storm_sewer Storm structures with rim/grate/invert elevations, pipe data TEXT on C-STRM layers, proximity grouped dwg_sanitary_sewer Sanitary manholes with rim/inverts, pipe sizes, flow direction TEXT on C-SSWR layers, proximity grouped dwg_water Water meters, hydrants, main sizes TEXT/INSERT on C-WATR layers dwg_property Property boundaries, bearings, acreage, owners, plat references TEXT on C-PROP layers, LWPOLYLINE boundaries dwg_other_utilities Gas, electric, telecom, field evidence notes TEXT on V-* and utility layers dwg_erosion_control BMP keynotes, erosion control features MTEXT on C-EROS/KYN layers dwg_grading Spot elevations, building pad, terrain analysis, pavement sections TEXT on C-SITE/C-BLDG layers dwg_contours Contour line data with vertex counts and elevation ranges LWPOLYLINE on C-TOPO layers Loading Into plans-spatial.json Read the existing plans-spatial.json, merge the new dwg_* sections (overwriting previous DWG data if re-processing), and write back. Include metadata: { "dwg_survey_data" : { "source_file" : "filename.dwg" , "extraction_date" : "2026-02-20" , "coordinate_system" : "Kentucky State Plane NAD83 (US Survey Feet)" , "total_points" : 598 , "elevation_range" : { "min" : 736.41 , "max" : 750.20 , "unit" : "ft" } , "points" : [ ... ] } } Also update project-config.json under documents_loaded to record the DWG extraction with a reference to which data sections were populated. Integration with Other Skills This skill feeds data to several other Foreman OS skills: quantitative-intelligence : DWG geometry is Priority 1 (exact) data source for quantity calculations project-data : All extracted sections follow the plans-spatial.json schema document-intelligence : DWG files detected during /process-docs should trigger this skill construction-takeoff : DWG survey points and contours provide ground-truth measurements When invoked from /process-docs or /set-project, this skill runs automatically for any .dwg files found in the project folder. The extracted data integrates seamlessly with data from other document types (PDFs, specs, schedules). Civil 3D Proprietary Object Limitations The DWG extraction pipeline uses libredwg (open-source) to convert DWG to DXF, then parse_dxf.py to extract entities. libredwg cannot decode Autodesk Civil 3D proprietary objects because they use undocumented internal formats. Affected Object Types Civil 3D Object DWG Entity Type What It Contains Impact Alignments AcDbAlignment Road centerlines, curve data, stationing Cannot extract road geometry or station values Surfaces AcDbSurface / AcDbTinSurface Terrain models, contours, spot elevations Cannot extract existing/proposed grade data Pipe Networks AcDbPipeNetwork Storm/sanitary pipe routing, structures Cannot extract pipe sizes, inverts, manholes from network objects Corridors AcDbCorridor Road cross-sections, assemblies Cannot extract road design details Parcels AcDbParcel Property boundaries, lot lines Cannot extract parcel data Profile Views AcDbProfileView Vertical alignments, profile grades Cannot extract profile/grade information Pressure Pipes AcDbPressurePipe Water distribution networks Cannot extract water system layout from network objects What DOES Work Standard DXF entities (LINE, POLYLINE, CIRCLE, ARC, TEXT, MTEXT, INSERT, ATTRIB) are fully supported Civil 3D drawings typically contain BOTH proprietary objects AND standard entities (labels, annotations, hatches) Layer names, text labels, and attribute data from standard entities are extractable even in Civil 3D files Block insertions (manholes, valves, hydrants as blocks) work normally Workaround — Export to Clean DXF from AutoCAD If the design team has AutoCAD Civil 3D, request a DXF export with these settings: In Civil 3D, use EXPORTTOAUTOCAD command (not SAVEAS DXF ) This "explodes" Civil 3D objects into standard AutoCAD entities: Alignments become polylines with station labels as text Surfaces become 3D faces or contour polylines Pipe networks become lines/circles with attribute data The resulting DXF is fully parseable by parse_dxf.py Alternative: Request PDF + DXF Combo Ask the design team for both DXF (for spatial data) and PDF plans (for annotations/details) document-intelligence extracts text/tables from PDFs dwg-extraction extracts geometry from the DXF Utility reconciliation (Pattern 7 in cross-reference-patterns.md) merges both sources Detection When parse_dxf.py encounters unknown entity types, it logs them as warnings: WARNING: Skipped entity type 'AcDbAlignment' at handle 0x1A3 (Civil 3D proprietary) WARNING: Skipped entity type 'AcDbTinSurface' at handle 0x2B7 (Civil 3D proprietary) If >20% of entities in a file are skipped proprietary objects, flag for superintendent with recommendation to request EXPORTTOAUTOCAD DXF from the design team. Limitations Civil 3D proprietary objects : See the detailed section above. In short, surfaces, alignments, corridors, and pipe networks stored as AcDb* objects are not decoded by libredwg. The skill extracts the standard DXF entities that Civil 3D writes alongside these objects, which typically contains all the annotation data (elevations, labels, keynotes). Use EXPORTTOAUTOCAD as a workaround. 3D solids : Complex 3D geometry (ACIS solids, meshes) is catalogued but not deeply parsed. External references (XREFs) : The skill processes the host DWG only. If critical data is in XREFs, they need to be bound or provided as separate files. Block definitions : Standard blocks are extracted by name and insertion point. Custom block geometry (internal entities) is not recursively parsed. Coordinate systems : The skill preserves whatever coordinate system the DWG uses. For Kentucky projects, this is typically State Plane NAD83. No coordinate transformation is performed.