How to Locate Control Points Hidden by Vegetation｜5 Practical Techniques to Reliably Find Them in the Field

On surveying sites, problems frequently occur where control points are covered by vegetation or brush and become invisible. There are likely many site personnel who have experienced starting surveying work only to find the essential control point nowhere to be found, spending half a day just searching for it. To reliably locate control points hidden by vegetation, it is important to combine advance preparation with systematic search procedures in the field. This article explains in detail how to find control points hidden by vegetation, focusing on five practical techniques that are actually useful on site.

Reference points are often placed in locations that are difficult to reach by hand, such as mountainous areas, farmland, and river embankments, and because vegetation growth changes dramatically with the seasons, a reliable search strategy is essential. If a reference point cannot be found, the entire operation can stall, creating risks of schedule delays and additional costs. By mastering the five techniques in this article, you can significantly reduce on-site search time and more reliably locate reference points.

Table of Contents

• Why do reference points get hidden by vegetation?

• Preparation: What to do before entering the site

• Technique 1: Coordinate-guided targeted search

• Technique 2: Cross-checking photos of the point record with the field

• Technique 3: Systematic on-the-ground search

• Technique 4: Removal of leaf litter and accumulated sediment, and metal detection

• Technique 5: Using seasonal timing to your advantage in searches

• Alternative actions if the reference point can't be found

• Use search records to improve future searches

• Revolutionize the efficiency of reference point searches with high-precision GNSS

Why do reference points become hidden by vegetation?

Understanding why reference points become obscured by vegetation leads to more efficient searches. The signs and survey markers for reference points are usually unobstructed when installed. However, after several years the surrounding vegetation grows, and in summer it is not uncommon for grasses to exceed 1 m (3.3 ft) in height. In mountainous areas away from urban centers or near the boundaries of farmland, lack of maintenance can allow woody plants to become overgrown and completely cover the reference point.

Visibility also varies depending on the type of reference point. Triangulation points and benchmarks managed by the Geospatial Information Authority of Japan sometimes have concrete survey stones installed almost flush with the ground, and they can be easily buried when grass grows tall. In contrast, metal markers and pin-type reference points are nearly level with the ground, so when covered by fallen leaves or accumulated sediment they become very difficult to spot. Furthermore, sediment inflow from past typhoons or heavy rains is another factor that accelerates the burial of reference points.

From spring through autumn, when surveying work is frequent, vegetation grows vigorously, and it is not uncommon for control points that were located the previous year to be unfindable the following year. Planning search operations that take seasonal vegetation changes into account can greatly affect work efficiency. If 10 or 20 years have already passed since a control point was installed, it is important to assume the surrounding environment has changed significantly when making the search plan.

Typical areas where concealment by vegetation tends to be a problem include the boundaries of fallow fields and abandoned farmland, along forest roads in mountain forests, the slopes of river embankments, and the corners of farmland. In these locations, farming activities and weed control often do not reach the areas around reference points, making it easier for vegetation to encroach. In addition, the edges of golf courses and parks, and planting strips along highways are also places where reference points tend to become difficult to see.

Preparations: What to Do Before Entering the Site

To efficiently locate reference points hidden by vegetation, preparation before entering the site is the most important step. If you neglect preparation, you will not only waste time on site, but you may also end up having your work interrupted without finding the reference point.

The first thing to do is collect recorded information about the control point in question. On the Geospatial Information Authority of Japan’s web service, you can view results information and observation notebooks (point records) for triangulation points and benchmarks. The point records include sketches and photographs showing the surroundings of the control point, containing very useful information for understanding the environment at the time of installation. By comparing photographs from the time of installation with current satellite imagery, you can to some extent grasp changes in vegetation growth.

Next, enter the coordinates of the reference point in advance into the GNSS device or the field app. If the coordinates are known, the GNSS can guide you to the reference point on site, greatly narrowing the search area. However, if the GNSS device’s positioning accuracy is on the order of meters (meter-level; several ft), you will still be searching within an error range of several meters (several ft), pushing aside vegetation as you go. With a high-accuracy device, you can narrow the search circle to within a diameter of several tens of centimeters (several tens of in).

It is also essential to prepare in advance the tools needed for the search. Clearing tools such as a grass sickle, pruning shears, and a folding saw; rakes and brooms to remove fallen leaves accumulated on survey markers; a small shovel for digging soil; and a metal rod for tapping the ground and locating survey markers by their resonance are useful. Protective gloves, goggles, and long-sleeved work clothes are also a must. In summer, bring insect repellent spray and an insecticide that can deal with wasp nests.

If the surveyor in charge is visiting a site for the first time, it can be useful to check in advance with local surveying firms or administrative officials and ask, "Do you know the current condition of this control point?" If local stakeholders have used the same control point in the past, you may obtain on-the-ground information such as "the grass has been overgrown recently" or "it was relatively easy to see last year."

Additionally, by asking owners of adjacent land and farmers who cultivate the fields, you may be able to learn about the current status of reference points. If a reference point is installed along a farm path or near the entrance to a forest road, nearby farmers or managers may be aware of its existence.

Technique 1: Coordinate-Guided Filtered Search

The most reliable and efficient technique is a GNSS-based coordinate-guided narrowing search. In this method, the known coordinates of a reference point are entered into the positioning device, and you approach the target location while monitoring the real-time difference between your current position and the target.

Because the positioning accuracy of typical smartphone GPS apps is on the order of several meters to around a dozen meters (several meters to around 12 m (39.4 ft)), the final search area tends to be wide. However, using high-precision GNSS can improve positioning accuracy to the centimeter level (cm level accuracy, half-inch accuracy), allowing you to narrow the target point down to within a 30 cm (11.8 in) radius. This dramatically reduces the area you need to search among vegetation and can greatly shorten search time.

When you approach directly above the reference point using coordinate guidance, proceed by slowly pressing the grass around your feet to check the firmness of the ground. If there is a survey marker you can often feel a hard sensation under the soles of your feet. Centered on that spot, inspect the ground surface within a radius of 1 m (3.3 ft) while removing grass.

When working alone, looking at the GNSS screen while moving tends to divert your attention from what’s ahead. Extra caution is required when working in thickets, where hazardous creatures such as insects or snakes may be present. Also, if the GNSS antenna is obstructed by vegetation, positioning accuracy can decrease, so in dense thickets it is recommended to perform positioning while standing in a location with an open view upward.

When using GNSS in a location surrounded by thicket where accuracy is unstable, it is advisable to step out of the thicket, take a position fix there, confirm the bearing and distance offsets to the target coordinates, and then re-enter the thicket. Even if GNSS is unstable inside the thicket, you can infer the target position from position fixes taken around the outer edge of the thicket.

Technique 2: Cross-checking photos of survey point records with on-site conditions

By matching the photographs and sketches recorded in the benchmark’s point record with what you find on site, you can infer the benchmark’s location even if vegetation is overgrown. The point record lists the direction and distance to major reference features as seen from the benchmark, and confirming those features on site provides clues to the approximate position of the benchmark.

For example, if there is a record stating "there is a utility pole to the north at a distance of 15 m (49.2 ft)", you would locate that pole on site, and the point 15 m (49.2 ft) south of it would become the center of the search. Corners of buildings, bridge railings, curbs, large rocks, and other man-made or natural features that tend to remain over time are often recorded as targets.

Also, by comparing the photographs in the survey record with the terrain and features on site, you can identify the topographic characteristics around the control point. Narrow the search area using topographic clues, such as the edge of a flat area located partway up a gentle slope. Cross-referencing satellite and aerial photographs with the site is also useful.

The point records may be old, and the features they reference may have been removed or relocated. In such cases, it is important to pay attention to the terrain itself (small hills, depressions, riverbank terraces, etc.). Terrain changes less than man-made features and can provide clues for locating control points over long periods.

Technique 3: Systematic Ground Survey

After narrowing the search area to some extent through coordinate guidance and cross-checking point records, a systematic ground survey is conducted. Rather than wandering around aimlessly, this method sets out a grid of sections at regular intervals and carefully inspects the surface one row at a time.

First, place flagging tape or flags around the perimeter of the search area to visualize the area. Next, walk along parallel lines roughly 50 cm (19.7 in) apart, searching for stone markers and metal markers by feeling with your feet and by visual inspection. In places where grass or fallen leaves have accumulated, it is also effective to lightly tap the ground with a metal rod to check for the feel of a stone marker. If there is a stone marker, you will hear a hard, concrete resonant sound.

If the grass has grown to waist level or higher, proceed by cutting a strip of uniform width with a sickle or pruning shears. This work is physically taxing, but it is essential for reducing the risk of missing reference points. Since grass-cutting in the midsummer heat also carries a risk of heatstroke, be sure to stay well hydrated and take regular breaks while working.

After exposing the ground, visually inspect the marker while paying attention to the shape and color of the monument. Triangulation point markers are made of mikage stone or granite and are grayish-white, while benchmarks are characterized by metal caps. If they have been buried for a long time, the surface is often covered with moss or mud, but lightly brushing it with a brush will reveal any engravings and distinctive worked surfaces.

Technique 4: Removal of Leaf Litter and Accumulated Sediment, and Metal Detection

If the reference point marker is made of metal (a metal marker or a pin), searching with a metal detector is very effective. A commercially available metal detector can detect metal markers even when they are buried under more than 10 cm (3.9 in) of soil or sediment. Humus tends to accumulate near the bases of vegetation, and metal markers can sometimes be buried about 5-10 cm (2.0-3.9 in).

When using metal detectors, having a lot of scrap iron and metal waste in the same area increases false positives. However, because the locations for placing reference points are carefully selected by people, they are often in places with relatively few scattered metal pieces, so metal detectors tend to be more effective there.

Metal detectors cannot be used for stone-marker–type control points, but methods that judge by the echo when lightly tapping the ground or by the feel underfoot are effective. Concrete or granite markers are harder than the surrounding soil, and you can detect a clear difference when you step on the ground. If the search area is small, another option is to confirm the location by thinly scraping away the surface soil with a shovel. However, it is important to work carefully so as not to damage the marker.

When removing accumulated fallen leaves, use a rake to gently sweep them to the side. After the ground appears beneath the leaves, pay attention to changes in the color and texture of the soil. Around the marker stone, the soil may have become compacted from years of placement, and that boundary can appear as a subtle difference in color.

Technique 5: Leveraging Seasonal Timing in Searches

The most commonly overlooked technique when searching for reference points hidden by vegetation is the choice of timing for the search. From winter to early spring (November–March), when vegetation is at its thinnest, deciduous trees drop their leaves and grasses die back, opening up visibility and making reference points easier to spot. Confirming the locations of reference points during this period will serve as useful reference information for subsequent field work.

When searching for reference points in summer, choosing the cool early morning hours can reduce the risk of heatstroke and insect bites. Also, the morning after rain the ground is wet and the grass tends to be flattened, so grass-cutting work may be easier.

Ideally, you should make it a habit to clear vegetation around control points once a year, but many sites are unable to do so because of management costs and the labor involved. At such sites, it is efficient to complete the clearing the day before surveying or first thing on the morning of the survey.

In some regions, there is a system that allows you to request maintenance and management around reference points from local collaborators or municipal governments. For national reference points, you may be able to check the management status or request regular mowing by contacting the Geospatial Information Authority of Japan or the Regional Development Bureau, and for reference points used in public surveying, by contacting the municipality that installed them.

Alternative measures when reference points cannot be found

If you cannot locate the control point even after trying all five techniques, you need to consider alternative measures. The first option to consider is using another control point nearby. If multiple control points have been established around the work area, you can proceed with the work from a different control point instead of the one you cannot find.

Another option is to obtain the required coordinates from other known points by polygonal surveying or intersection methods. However, this approach requires careful consideration of accuracy, and it is necessary to confirm in advance whether it can meet the accuracy requirements specified in the design documents.

There is also a method called standalone RTK positioning that uses network RTK or correction information services. Network RTK that uses electronic reference points as reference stations is a method that can directly obtain coordinates with centimeter-level accuracy (cm level accuracy (half-inch accuracy)) without using control points. This method is particularly effective as an alternative for work in mountainous areas and farmland where control points cannot be found.

If the loss or damage of control points is confirmed in accordance with standards such as the Public Surveying Work Regulations, reporting to the competent authority and filing an application for restoration may be required. In particular, do not fail to notify the managing agency regarding national control points (triangulation points and benchmarks). Properly completing the loss-reporting procedures will help ensure the accuracy of subsequent work and records.

Use records of reference point searches next time

When you discover a reference point, it is important to make sure you record that information. Doing so can greatly improve work efficiency when using the same reference point next time. Information that should be recorded includes the discovery conditions of the reference point (condition of vegetation, degree of burial), the positional relationship to surrounding landmarks and features, photographs taken at the time of discovery (from multiple directions), and the condition of the benchmark stone (tilt, wear, damage).

Share recorded information within the company so that the person who will be assigned to the same site next time can refer to it. Don’t leave it as an individual’s experience; accumulating it as team knowledge directly leads to improved on-site productivity. Organizing information using cloud storage and surveying operations management systems will greatly help future exploration work.

Useful information to record includes the date and time when the reference point was found, the surrounding grass height and vegetation condition at that time, the ground moisture and degree of muddiness, and the time required to find it. With this information, you can identify seasonal trends in how easy reference points are to locate and use that insight to plan future work. In particular, for projects that occur at the same time each year, records from the previous year make it possible to determine the optimal timing for the work.

Also, recording the time and effort required to locate reference points can be used to improve future schedule management. By enabling appropriate estimates of the time needed for reference point searches during the quotation stage, more accurate delivery-date predictions become possible.

Revolutionizing the Efficiency of Reference Point Search with High-Precision GNSS

In recent years, the adoption of high-precision GNSS devices at surveying sites has rapidly accelerated. Compared with conventional surveying instruments, compact, easy-to-carry devices that enable single-person on-site work have emerged.

Among these, a high-precision GNSS device that attaches to a smartphone is attracting attention. By attaching a dedicated GNSS antenna to a smartphone, an existing smartphone can be used as a positioning terminal with centimeter-level accuracy (half-inch accuracy). Even at sites where vegetation is dense and control points are not visible, because you can be guided precisely toward the coordinates of the control point while looking at the smartphone screen, search time can be greatly reduced.

High-precision GNSS-enabled positioning devices are precisely the kind of equipment that meets these field needs. RTK positioning enables obtaining the current position with centimeter-level accuracy (inch-level accuracy), and when combined with a guidance function to pre-registered control point coordinates, you can proceed with searches without getting lost even in vegetation-covered sites. They are designed for solo field work and are useful in a wide range of applications, such as surveying sites in mountain areas where heavy machinery or large equipment is difficult to bring in, and verifying agricultural land boundaries. What used to take 2-3 people half a day to find control points can, by utilizing such high-precision devices, be completed by one person in a short time. If you want to fundamentally streamline on-site control point searches, please consider introducing such devices.

Especially at sites with limited transportation access, such as remote islands or deep mountain areas, reducing travel time by shortening search time has a significant impact on operational efficiency. High-precision GNSS devices that can efficiently locate multiple control points and enable solo work are a powerful tool for realizing workstyle reforms in surveying operations.