Frequently Asked Questions

A draft survey is a method to determine the weight of cargo loaded or discharged from a vessel by measuring the vessel's draft (how deep it sits in the water) before and after loading. It is critical for commercial settlement, customs declaration, and trade compliance. GOTEC's AI-powered approach replaces risky manual boat-based observation with machine vision and semantic segmentation, achieving millimeter-level accuracy. Explore our draft survey equipment to see how we reduce traditional 2-4 hour surveys to under 30 minutes.

A draft survey determines cargo weight by measuring how much water a vessel displaces before and after loading or discharge. It works on Archimedes' principle: the weight of water the ship pushes aside equals the ship's weight. You measure the draft (depth) at six points — forward port and starboard, midship port and starboard, aft port and starboard — before and after cargo operations. The difference in displacement, after correcting for ballast water, fuel, fresh water, and other variables, gives you the cargo weight.

The six readings are averaged, then corrected for two things: where the draft marks sit relative to the perpendiculars (they're often not exactly at them), and hull deformation (hog or sag — the ship bending up or down in the middle). The corrected averages feed into a formula called the Quarter Mean Draft, which weights the midship reading six times higher than bow and stern to minimize hog/sag distortion. From there, you look up the displacement in the ship's hydrostatic tables, apply trim and density corrections, subtract the deductibles (ballast, fuel, fresh water, constant), and what's left is your cargo figure.

A well-executed draft survey has an accepted accuracy of ±0.5% of cargo weight. On a 100,000-tonne bulk shipment, that's ±500 tonnes. Getting it right matters — a 0.5% error on a Capesize iron ore cargo at $100/tonne is a $50,000 settlement dispute. This is why the IMO, UNECE, and major P&I clubs all require standardized draft survey procedures for bulk cargo transactions.

Draft marks are painted on the hull at the bow, midship, and stern on both port and starboard sides. You need all six readings. Each mark shows the depth from the keel in decimetres (10 cm intervals) using Roman-style numerals or Arabic numbers, typically 10 cm tall with 10 cm gaps between them. The bottom of each digit marks the even-decimetre depth.

How to read them:

1. Get as close to the waterline as possible — ideally in a small boat or from a quay at water level. Looking down from the deck introduces parallax error.
2. Identify which digit is at or just below the waterline. If the waterline cuts through the middle of the "8" on a mark reading 12.8M, the draft is 12.84 metres (4/10 of the way through a 10 cm digit).
3. Read to the nearest centimetre. Experienced surveyors consistently read to ±1 cm accuracy in calm conditions.
4. If there's swell, take 20 rapid readings, discard the 2 highest and 2 lowest, average the remaining 16.
5. Read all six positions quickly — water density changes with tide and temperature can shift readings over time.

After reading: average port and starboard at each position to get Fore Mean, Mid Mean, Aft Mean. Calculate apparent trim (Aft Mean minus Fore Mean). Apply draft mark corrections if the marks aren't at the perpendiculars (most ships have this documented in their stability booklet). Use the 6-sided Quarter Mean formula — (6 × corrected midship + corrected fore + corrected aft) ÷ 8 — to get the draft value for hydrostatic table lookup. This formula specifically dampens the effect of hull bending, which is why the midship gets 6x weight.

Common mistakes: reading the top of a digit instead of the bottom (adds ~10 cm per mark — a 60-tonne error on a Panamax); forgetting to note which side the marks are on when the ship has a slight list (port and starboard will differ); and assuming the draft marks are at the perpendiculars when many ships have them offset by several metres.

GOTEC combines HD cameras with computer vision algorithms. Our semantic segmentation neural networks separate waterline and scale digits, then compute accurate draft values using wave-resistant curve fitting and perspective tilt correction. The system outputs traceable digital records suitable for settlement and regulatory review. Learn more about our visual AI algorithms that power millimeter-level maritime measurement.

GOTEC offers three main product lines:

• Ship Draft Observation Instruments, 4K machine-vision capture with optical anti-shake, remote pan-tilt control, and 30m+ transmission support.
• Ballast Water Level Meters, Portable smart meters with ±5mm precision, one-click operation, and 10,000+ local records.
• Remote Inspection Platforms, Low-latency video collaboration with infrared support and file sharing capabilities.

GOTEC provides four core algorithm services:

• Draft Scale Recognition, Semantic segmentation for waterline/scale digit separation with wave-resistant fitting
• Ballast Level Estimation, Sensor fusion with rule models for millimeter-level processing
• Container Information Recognition, OCR for container ID, type, and related field data from images/video
• Seal Number OCR, Target detection + OCR pipeline for customs seal verification and anti-tampering records

Explore our full suite of AI algorithms designed for port and customs operations.

Customs authorities gain several key benefits:

• Reduced safety risks by replacing manual boat-based observation
• Higher throughput under growing trade volume with automated workflows
• Full transparency and replayability of digital measurement records
• Improved intelligent supervision image and compliance posture

Discover our smart customs solutions built for end-to-end digital supervision.

GOTEC (Shandong) Equipment Technology Co., Ltd. is headquartered at No. 68 Changsha Road, Shibei District, Qingdao, China. We serve ports, customs authorities, inspection agencies, and cargo owners globally, with deployed solutions across multiple major Chinese ports and growing international presence.

GOTEC's portable ballast water level meter achieves ±5mm precision sensing. It uses high-sensitivity sensor streams combined with rule models to validate touch-water and touch-bottom states, producing reliable tank-level results with one-click operation and automatic data storage for 10,000+ local records. Browse our full range of port measurement instruments, including draft observation and ballast measurement devices.

Yes. GOTEC designs its platform for integration. Our cloud connectivity, API access (available on PRO and Enterprise plans), and support for standard data formats allow integration with weighbridge systems, customs data platforms, and port management software. Contact our team to discuss your specific integration requirements.

Timelines vary by scope. Pilot projects with a single product line typically deploy in 2-4 weeks. Full-scale deployments covering multiple terminals or integrated draft survey + container weighing solutions typically take 2-4 months, including on-site installation, integration, and operator training.

GOTEC employs end-to-end encryption for data transmission, role-based access controls, and full audit trail capabilities. Our Enterprise plan includes SLA-backed security reviews and audit support. All measurement data is traceable from capture to sign-off, supporting regulatory compliance requirements for customs and trade.

GOTEC provides comprehensive support: on-site installation and commissioning, operator training for field teams, technical documentation and integration guides, email and phone support, and optional named technical account management for Enterprise customers. We work alongside operations teams to ensure digital tools deliver real efficiency.

You can reach GOTEC at or call +86 18661801001. Fill out the contact form on our Contact page with your requirements, and our sales team will respond within 1 business day with a tailored quotation. We offer pilot project evaluations and on-site demonstrations for qualified opportunities.