TMH 18 explained: South African road asset data exchange formats

TMH 18 is the South African road manual that sets out the file formats for submitting a road network and its condition data to a national roads database (COTO, 2014) (COTO, 2016i). It exists as two drafts, Draft Version 4 (October 2014) and Committee Draft 5.0 (February 2016). TMH (Technical Methods for Highways) is a series of South African road manuals compiled under COTO, the Committee of Transport Officials.

Key facts

• TMH 18, the South African road asset data exchange manual, exists as Draft Version 4 (October 2014) and Committee Draft 5.0 (February 2016); the Department of Transport lists both files.
• Every TMH 18 tabular data file is semicolon-delimited UTF-8 text that needs no header row, and a record with a missing field causes the file to be rejected (section 1.1).
• TMH 18 renames the four visual condition extensions between Draft Version 4 (.vcf, .vcc, .vcb, .vcu) and Committee Draft 5.0 (.flx, .con, .blc, .vgi).
• TMH 18 Draft Version 4 describes road geometry as KML or KMZ, while Committee Draft 5.0 describes GML with its schema file or a shapefile.
• In TMH 18 Committee Draft 5.0, Table 5 lists F25 deflectometer data under .fwd, whereas Draft Version 4 uses .fwd for normalised deflection bowls and .f25 for F25 data.
• TMH 22 (Draft, March 2013) and TMH 13 Part B cite 'TMH 20' for the formats TMH 18 sets out, but the Department of Transport catalogue has no TMH 20.

What is TMH 18, and why are there two editions?

TMH 18 is titled Road Asset Data Electronic Exchange Formats (COTO, 2014). It describes files, not measurement: survey method belongs to TMH 13 (Committee Draft Final, May 2016), the South African manual on automated road condition assessments.

Two editions are in circulation, Draft Version 4 (October 2014) (COTO, 2014) and Committee Draft 5.0 (February 2016) (COTO, 2016i), and the Department of Transport's catalogue lists both under different titles (Department of Transport, n.d.). Both are drafts, and the front matter of Committee Draft 5.0 states that committee drafts have no legal standing (COTO, 2016i). Neither edition says it replaces the other, so write the edition and date in full.

Which rules apply to every TMH 18 data file?

Both editions open with the same global rules in section 1, which govern the tabular data files, not the geometry or raw F25 files (COTO, 2014) (COTO, 2016i). A data file is plain UTF-8 text in the style of comma-separated values, with a semicolon replacing the comma. Each record ends with a carriage return and a line feed. No header row is required, because the layouts are predefined. An empty field is shown by its delimiter alone, and a record with a missing field causes the file to be rejected. A file name joins an authority code, a road identifier and a generation date and hour, then the extension, which is case sensitive.

Every record places itself on the network with a road identifier, a lane code where the data describe one lane, start and end kilometres (or one survey point), and the survey date. Section 5 states that values outside the road's start and end are discarded.

Narrated film · 1:36

What each TMH 18 file carries

A radial map puts the network definition at the centre with the TMH 18 file families around it, then compares Draft Version 4 and Committee Draft 5.0 side by side.

Read the film transcript

In the South African road manual TMH 18, one file extension can mean two things. TMH 18 sets out the file formats for submitting road network and condition data. It exists as Draft Version 4, from October 2014, and Committee Draft 5.0, from February 2016. At the centre is the network definition, one row per road, which every other file points back to. Around it are geometry, classification, lanes, traffic, visual condition, combined instrument and falling weight deflectometer, or FWD, files. Draft Version 4 describes geometry as KML or KMZ, and Committee Draft 5.0 as GML or a shapefile. The visual condition extensions are renamed, and Committee Draft 5.0 adds an authority code to every record layout. In Draft Version 4, the FWD extension holds normalised deflection bowls. In Committee Draft 5.0, Table 5 lists raw F25 data under that same extension. Both editions label deflections in nanometres, while TMH 22 and TMH 13 Part F use micrometres. So a synthetic reading of 450 is 0.45 micrometres as nanometres, but 0.45 millimetres as micrometres. TMH 18 describes files, whereas TMH 13 and TMH 9 describe measuring and rating. Some manuals cite "TMH 20" for these formats, but the Department of Transport catalogue lists none. Name the edition and date, and agree in writing what each extension and unit means. Next, see TMH 13 for the measurements and TMH 22 for the system receiving the files.

Which file types does TMH 18 define, and what does each carry?

Every TMH 18 file points back to a road in the network file, the reference system for all other data (section 2.1) (COTO, 2014). The families are:

• Network definition (.net). One row per road, with its start and end km and the dates the definition is valid. A dual carriageway is two roads, and a change of alignment is resubmitted as a new version that data map to by survey date (sections 2.1 and 2.2.2).
• Geometry. The line along which kilometres are measured. Section 2.2.1 asks for "a maximum resolution of a point every 20 metres" and calls 10 m adequate.
• Road classification (.rcl). A new segment each time an attribute changes, such as road type, width or lane count (section 3.1).
• Lanes (.lan). Lane and shoulder widths per segment. A lane code joins direction, lane type and lane position, numbered from the fast lane, so P1 is the fast lane in the primary direction (sections 4.1 and 4.2).
• Traffic (.adt). Average annual daily traffic by vehicle class (section 4.4).
• Visual condition summaries. One record per assessed segment, with one file type for each pavement family: flexible, concrete, block and unsurfaced. Sections 5.2 to 5.5 refer to Parts B to E of TMH 9, the South African manual for visual assessment of road pavements, and name no edition; the latest is Committee Draft Final, May 2016.
• Combined Instrument Data (.cid). The international roughness index (IRI) in m/km, rut depth and mean profile depth (MPD), each with standard deviations, in one record that shares road, lane, start, end and date (section 5.6). See how a profiler run becomes a register value.
• Deflection (.fwd and .f25). Falling weight deflectometer (FWD) data, with F25 as the raw data format, covered below.

Here .cid means the TMH 18 Combined Instrument Data file, not the IEC 61850 substation standard's CID files.

How do TMH 18 Draft Version 4 and Committee Draft 5.0 differ?

TMH 18 Draft Version 4 and Committee Draft 5.0 differ in file extensions, geometry format, attributes and record layouts. Table 5 of each edition lists the extensions (section 1.6) (COTO, 2014) (COTO, 2016i).

File family	Draft Version 4 (October 2014)	Committee Draft 5.0 (February 2016)
Network, classification, lane, traffic and combined instrument	.net, .rcl, .lan, .adt, .cid	same extensions
Road geometry	.kml or .kmz	.gml with .xsd, or .shp with .shx, .dbf and .prj
Visual condition, flexible	.vcf	.flx
Visual condition, concrete	.vcc	.con
Visual condition, block	.vcb	.blc
Visual condition, unsurfaced	.vcu	.vgi
Normalised FWD bowls	.fwd	none named in Table 5
FWD F25 raw data	.f25	.fwd in Table 5; .f25 in section 5.7.1

Geometry format. Draft Version 4 describes Keyhole Markup Language (KML 2.2), also compressed as KMZ. Committee Draft 5.0 describes Geography Markup Language (GML 3.1.1) with a schema file, or a shapefile, and states that coordinates run in the direction of the start and end km (section 2.3). The insight on chainage, direction and wheelpath explains why that direction matters.

Attributes. In Committee Draft 5.0 the network definition carries the surface type and relates one to one to a geometry feature, so a road is split wherever those attributes change. In Draft Version 4 the surface type sits in the road classification file.

Record layouts. With no header row, the layout is known only from the edition. Committee Draft 5.0 adds an authority code field to every record layout (Draft Version 4 has one only in the network definition), lengthens the road identifier field and renames columns: the measurement date of the combined-instrument file is MEASURE_DATE in Draft Version 4 and TEST_DATE in Committee Draft 5.0. The flexible-pavement visual file lists degree and extent from 1 to 5 in Draft Version 4 and from 0 to 5 in Committee Draft 5.0 (section 5.2).

What does a TMH 18 .fwd file hold, and in which unit?

The .fwd extension in TMH 18 has two meanings: normalised deflection bowls in Draft Version 4 and F25 data in Committee Draft 5.0. In Draft Version 4, a .fwd file holds the bowls (section 5.7): survey point, temperatures, load, drop number, deflections at fixed offsets from the load plate, pulse duration, and the base, middle and lower layer indices (BLI, MLI and LLI). Raw FWD output goes in a separate .f25 file (section 5.8) (COTO, 2014). In Committee Draft 5.0, Table 5 lists F25 data under .fwd, section 5.7 names no extension for normalised bowls, and section 5.7.1 still describes an .f25 extension (COTO, 2016i). Agree in writing what a .fwd file carries.

A deflection value also needs a unit. Both editions label the deflection columns nanometres, while TMH 22 (Draft, March 2013), the draft South African road asset management manual, states that deflections are reported in micrometres (section D.4.5) (COTO, 2013), and TMH 13 Part F gives sensor ranges in micrometres (Table F.1) (COTO, 2016g). The units differ by a factor of 1,000. Without a header row, state the unit on the definition sheet that travels with the file; TMH 13 Part C (section C.5.3) asks the contractor to supply such a sheet for codes, abbreviations and column naming (COTO, 2016a). See how FWD deflection data travels between manuals.

Why do some manuals cite TMH 20 for the TMH 18 formats?

The label "TMH 20" points at the formats TMH 18 sets out, and the Department of Transport catalogue lists no TMH document numbered 20 (Department of Transport, n.d.), so cite TMH 18 with its edition. TMH 22 lists TMH 18 as COTO Data Exchange Formats but, in a separate entry, gives the title Standard for Road Network Data to a number 20 (section A.2), and its Part D sends the reader to "TMH 20" for reporting formats (section D.4) (COTO, 2013). TMH 13 Part B does the same (section B.4.2) (COTO, 2016c), while TMH 13 Part C names TMH 18 (section C.5.3) (COTO, 2016a). The title Standard for Road Network Data is the running head of Draft Version 4 (with COTO in front) and the catalogue title of Committee Draft 5.0 (COTO, 2014) (Department of Transport, n.d.).

What does a TMH 18 .cid file look like under each edition?

Synthetic example: every value is invented. A crew records a 100 m section of synthetic road S1, for authority code TST, from 3,200 m to 3,300 m on 21 October 2026 at 08:00. The file is generated at 14:00, and its name carries that hour (section 1.5): TST_S1_2026102114.cid. Inside it, the positions are written in kilometres to three decimals: 3.200 and 3.300, not 3200 and 3300 (section 1.7) (COTO, 2014).

The first fields differ by edition. Under Draft Version 4 (section 5.6) they run road, lane, start, end: S1;P1;3.200;3.300. Under Committee Draft 5.0 (section 5.6) the authority code comes first: TST;S1;P1;3.200;3.300 (COTO, 2016i). Software built for Draft Version 4 finds TST where it expects the road and P1 where it expects a start km, so it rejects or misreads the record.

The same crew reports a centre deflection of 450. As nanometres, that is 0.45 micrometres; as micrometres, it is 0.45 millimetres. Only the unit on the definition sheet settles which one the sender meant.

What does TMH 18 not decide, and what is commonly misread?

TMH 18 does not say how a quantity is measured or rated; TMH 13 and TMH 9 do. It gives the visual condition index (VCI) for road pavements a numeric format and no formula, so name the VCI variant beside the file. It does not say which normalisation a bowl has had; TMH 13 Part F describes normalising to a target load and, in Appendix F-2, to a reference asphalt temperature (COTO, 2016g). A file in the right layout shows the layout was followed, not that the survey was sound: see the pre-send checks and the insight on survey acceptance evidence.

Common misreadings:

• ".cid is TMH 18." The .cid file is one file type in section 5.6. TMH 18 is the whole family.
• "Cite the table number." Draft Version 4 numbers that table differently in its contents list and its body, so cite section 5.6.

What should you keep when exchanging TMH 18 files?

Keep, for each exchange:

• the edition and date in full, and the receiver's written confirmation of it;
• the file families and extensions delivered, with the agreed meaning of .fwd;
• the definition sheet: units, codes, column names and the VCI variant;
• the network definition version and dates, and the geometry file with its coordinate direction;
• the generation date and hour in each file name and the survey date in the records;
• the receiving system's acceptance or rejection message.

Frequently asked

Is TMH 18 also called TMH 20?

Not as a separate document. TMH 22 (Draft, March 2013), the South African road asset management manual, and TMH 13 Part B send readers to 'TMH 20' for the data formats that TMH 18, the South African road data exchange manual, sets out, although TMH 22 also lists TMH 18 separately. The Department of Transport catalogue has no TMH 20 entry, so cite TMH 18 with its edition and date.

What is the difference between TMH 18 Draft Version 4 and Committee Draft 5.0?

TMH 18 Draft Version 4 (October 2014) and Committee Draft 5.0 (February 2016) differ in the visual condition extensions, the geometry format (KML or KMZ against GML or shapefile), the record layouts, where Committee Draft 5.0 adds an authority code, and the meaning of .fwd. TMH 18, also written TMH18, is the South African manual for road asset data exchange files.

What does a .fwd file mean in TMH 18?

In TMH 18 Draft Version 4 a .fwd file holds normalised falling weight deflectometer bowls, and raw F25 data uses .f25. In Committee Draft 5.0, Table 5 lists F25 data under .fwd and names no extension for normalised bowls, so the sender and the receiving authority should agree in writing what a .fwd file carries.

Is a .cid file the same thing as TMH 18?

No. The .cid file is one file type in TMH 18, the South African road asset data exchange manual: the Combined Instrument Data file for road roughness, rutting and texture described in section 5.6. TMH 18 defines the whole family of road network and condition files. The .cid file is also unrelated to the CID configuration files of the IEC 61850 substation standard.

Which unit do TMH 18 deflection values use?

Both TMH 18 editions label the deflection columns nanometres, but TMH 22 (Draft, March 2013) section D.4.5 says deflections are reported in micrometres, and TMH 13 Part F gives sensor ranges in micrometres. The units differ by a factor of 1,000, so state the unit on the definition sheet that accompanies the file.

Does TMH 18 say how to measure road roughness or rate pavement distress?

No. TMH 18 describes files, not measurement. TMH 13, the South African manual on automated road condition assessments, covers how a survey is carried out, and TMH 9, the South African manual for visual assessment of road pavements, covers the visual ratings. TMH 18 gives the visual condition index (VCI) for road pavements a numeric format and no formula, so name the VCI variant beside the file.

Where this connects

Two manuals come next. TMH 13 describes how the quantities in the combined-instrument and deflection files are measured. TMH 22 describes the road asset management system (RAMS) that receives the files, and its section D.5.2 says the COTO road network data standard should be used for all transfer of data (COTO, 2013).

References

1. COTO (2014). TMH 18: Road Asset Data Electronic Exchange Formats. Draft Version 4, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
2. COTO (2016i). TMH 18: Road Asset Data Electronic Exchange Formats. Committee Draft 5.0, February 2016. Original source.Sections, tables and clauses are cited inline in each guide
3. COTO (2013). Draft TMH 22: Road Asset Management Manual. Draft, March 2013. Original source.Sections, tables and clauses are cited inline in each guide
4. COTO (2016g). TMH 13: Automated Road Condition Assessments. Part F: Surface Deflection. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
5. COTO (2016a). TMH13: Automated Road Condition Assessments. Part C: Roughness. Committee Draft Final, May 2016. Original source.Sections C.2–C.5; Table C.5 is for calibration-section selection
6. COTO (2016c). TMH13: Automated Road Condition Assessments. Part B: Positioning. Committee Draft Final, May 2016. Original source.Positioning and distance measurement guidance
7. Department of Transport (n.d.). Roads: Engineering Standard Guidelines and Manuals. Official catalogue; accessed 5 September 2026. Original source.TMH22 V8b Draft; TRH12; TRH15; TRH24 catalogue entries

Use the manual edition and project requirements that apply to your work. Worked examples use synthetic values and are not site-specific design advice.