Steps to Efficiently Locate Invisible Reference Points｜A Comprehensive Guide to Countermeasures for Vegetation, Seasons, and Terrain

On surveying sites, the task of locating reference points that are invisible or difficult to see is unavoidable. Not only concealment by vegetation, but seasonal changes in plant growth and topographical conditions also greatly affect the visibility of reference points. In this article, from the perspective of "efficiently locating invisible reference points," we thoroughly explain specific countermeasures and procedures for each factor—vegetation, seasons, and terrain. The aim is to provide practical content that field personnel can refer to when the need arises.

Delays in the construction schedule and increased costs caused by the inability to find reference points significantly affect the overall profitability of a project. Therefore, combining systematic search procedures and the use of tools, rather than relying solely on the skills and experience of individual surveyors and site personnel, leads to sustainable site operations.

Table of Contents

• Three main causes of invisible reference points

• Procedures to counter visibility loss caused by vegetation

• Seasonal considerations and optimal timing for locating reference points

• Terrain-specific search strategies

• Record-keeping to prevent repeated searches at the same site

• Utilizing equipment to speed up discovery of reference points hidden by vegetation

• An integrated approach to reliably locate invisible reference points

Three primary causes of invisible reference points

The causes that make reference points difficult to see can be broadly classified into three categories. Correctly understanding each cause leads to appropriate countermeasures.

The first is concealment by vegetation. Herbaceous plants (grasses) and woody plants (shrubs and trees) can grow over benchmark stones or metal markers of reference points, physically covering them so they become invisible. Especially in summer, grass grows rapidly and a reference point can be completely buried within one month. Accumulation of fallen leaves and humus also contributes to concealment by vegetation. This is particularly pronounced on roadside slopes and in the corners of agricultural fields, locations where regular weeding is difficult.

The second is burial due to sediment and water. After heavy rain or floods, sediment can wash in and cover reference points. In farmland, soil can be moved during agricultural work, causing reference points to become buried. In snowy regions, reference points may be covered by snow in winter, making them difficult to locate in the first place. Reference points installed near rivers or at the lower parts of slopes are at particularly high risk of burial from sediment accumulation.

The third is terrain-induced poor visibility. If a reference point is installed partway up a slope or in a hollow, it can be harder to find than on flat ground. When the surrounding terrain is complex and there are few landmarks, accurately pinpointing the location also becomes difficult. In the complex terrain of mountainous areas, even if you have coordinate values, it can be hard to determine "where exactly that place is."

These causes can occur in combination; for example, in a situation where 'vegetation is dense, sediment has accumulated, and the location is midway up a slope', even an experienced surveyor will take time to discover it. It is important to prepare procedures and tools that can address each of these factors.

By predicting in advance when a control point will be difficult to see, on-site responses can be made more efficient. If you can judge from the point records and satellite photos that "this control point will be hard to find," you can prepare the necessary equipment beforehand and allow extra time. The ability to predict how easy a control point will be to find improves with accumulated experience. This leads to professional on-site management.

Procedures for mitigating reduced visibility caused by vegetation

This guide explains the procedures to address cases where vegetation is the primary reason a reference point cannot be seen. Dealing with vegetation is basically done by "brush cutting", but it is important to follow the correct procedures to improve efficiency.

First, before entering the site, input the coordinates of the reference point into the GNSS device or app. Use coordinate guidance to approach within a radius of several meters (several ft) of the target point, and from there begin mowing the grass in a circular pattern. It is inefficient to mow a wide area from the start. By beginning from the area narrowed down by coordinate guidance, you can minimize work time. With a high-precision GNSS device, it is possible to narrow the radius down further to several tens of centimeters (several in). When entering coordinates, be sure to check the geodetic datum and coordinate format in particular, and take care not to start searching with incorrect coordinates. Having multiple people double-check helps prevent on-site mistakes.

Trimming should be carried out from the outside toward the inside. Rather than moving straight toward the center of the thicket, it is safer and more efficient to narrow the area gradually from the perimeter. When using a brush cutter (grass trimmer), be aware of flying debris and wear protective gear. Hand work with a sickle or pruning shears takes more time but allows for precise work. During trimming, always check that the surrounding area is safe, and ideally station a colleague to watch for other people or for dangerous objects (stones, sticks, etc.) being thrown.

To improve the efficiency of brush-cutting work, it is important not to skip inspecting the brush cutter and tools before starting. For engine-powered brush cutters, check that the fuel tank is full, and maintaining a sharp blade will help the work proceed smoothly. Also, if the work period is long, rotating multiple workers will effectively reduce fatigue and improve safety.

When the ground is exposed, visually inspect it again. It is important to know the appearance corresponding to the type of control point—concrete marker stones, metal pins, metal plates, etc. Marker stones for national control points are generally made of mikage stone or granite, and the top is often engraved with inscriptions such as "triangulation point" or "benchmark." Metal markers are iron discs that often have a "+" mark in the center. The surface of a marker stone usually loses its luster and is often soiled with dirt or moss, but it is easier to find because it is harder and differs in color from the surrounding soil. By checking reference materials in advance about the types and appearances of control points, field verification will be more reliable and quicker.

If removing the grass doesn't reveal it, try removing a thin layer of leaf litter and surface humus. Use a shovel or rake to remove about 5 cm (2.0 in) of topsoil, and buried marker stones or metal markers may appear. Tap the ground with a metal rod to find spots that give a hard, resonant sound, and prioritize digging at those locations. When excavating, to avoid damaging the marker stone, keep the shovel blade turned sideways and carefully dig it out. In most cases deep digging is not necessary, and because the risk of damaging the marker increases with depth, you should generally limit excavation to a shallow depth.

If plant roots are entwined around the survey marker, cut the roots first and then expose the marker. Because forcibly pulling the roots may move the marker, cut the roots with scissors or a knife and then carefully remove them. If it is covered by thick vines such as ivy or kudzu, have several people carefully cut the roots while removing the vines.

Precautions and Optimal Timing for Seasonal Reference Point Searches

Because the condition of vegetation changes greatly with the seasons, the difficulty of searching for reference points also varies by season. Knowing the characteristics of each season and timing searches optimally improves efficiency.

Spring (March–May) is the period when grass begins to grow. Early spring (March–April) is when last year’s dead grass remains and new shoots start to appear, and because grass height is still low it is a relatively easy time to spot them. However, the ground is often wet from snowmelt, so be careful of muddy conditions. Around Golden Week, weed growth accelerates rapidly, so from May onward the difficulty becomes similar to summer. In mountainous areas there can still be lingering snow even in late May, so caution is required. The degree of ground moisture in early spring varies greatly depending on region and elevation, so checking local conditions in advance is important when planning.

Summer (June to September) is when vegetation growth is most vigorous and control point searches are most difficult. Grass height can reach 1-2 m (3.3-6.6 ft), requiring extensive brush-cutting work. Risks such as heatstroke, insect bites, and snakes also increase. However, because surveying work is most frequent in this season, thorough preparation and procedures are essential. In summer searches, working in the early morning (around 6-9 a.m.) is most efficient and safe. The goal is to complete brush-cutting before temperatures rise and finish precision searches using high-precision GNSS in the morning. In particular, during the rainy season (late May to July) humidity is high and working in the undergrowth becomes even more difficult, so it is desirable to avoid field work during this period.

Autumn (October–November) is the period when vegetation begins to wither, and reference points gradually become easier to see. In late autumn (from November onward), the leaves of deciduous trees fall and visibility opens up, greatly reducing the difficulty of searches. During autumn search operations, simply removing dead grass often reveals reference points. However, after clearing vegetation, moving the cut grass to another location makes the ground surface easier to see. During the autumn rainy season (late September to early October), the ground can become muddy, so working in rubber boots rather than sneakers is safer.

Winter (December–February) is the season when control points are most easily visible. Deciduous trees shed their leaves and annual plants die back, making the ground surface much more visible. In areas with little or no snowfall, it is ideal to verify the positions of control points during this period to prepare for the following year’s surveying work. However, in snowy regions they can be buried by snow. During winter verification work, placing a pole or survey flag at the control point location makes them easier to locate during the next fieldwork. Because temperatures are low in winter, appropriate cold-weather clothing is essential to prevent workers from losing body heat and to avoid frostbite of the hands and fingers.

Terrain-specific Search Strategies

You need to adapt your search strategy to the characteristics of the terrain where reference points are installed. Accurately interpreting the terrain leads to more efficient discovery of reference points.

On flat farmland and developed land, grass is often the main obstacle. After mowing, the entire ground becomes clearly visible, making visual inspection easier. However, because soil in farmland is easily moved by agricultural work, keep in mind that reference points may be somewhat buried. Also, corners of fields and the ridges or paths between paddies tend to have dense grass growth and require especially thorough mowing. For reference points installed in rice paddies, the best time to search is from winter to early spring when the water has been drained.

In mountainous and sloping areas, soil and debris tend to move along the slope, and benchmarks can be covered with soil on the downslope side. Also, plant roots often become entangled with the benchmark stone, making removal work complicated. By searching from an overhead vantage point, you can estimate the location of the benchmark from topographical features (flat areas, traces of artificial leveling, etc.). Work on steep slopes carries the risk of slips and falls, so make safety your top priority. When searching on slopes, work in a two-person team, with one person observing from above and the other working below.

Along rivers and waterways, flowing water during high stages easily deposits sediment, and reference points may be deeply buried. Also, waterside grasses grow vigorously, so vegetation tends to be dense overall. In moist soils the ground may be soft, and survey markers may be tilted or have settled. Work on riverbeds and along waterways can be dangerous during high water, so pay attention to weather and water levels. Reference points on river embankments require care during removal so as not to damage the embankment’s stability. It is advisable to consult the embankment manager in advance.

In forests near urban areas, the ground under coniferous stands such as Japanese cedar and cypress may at first seem easy to see, but there is often a thick accumulation of dead leaves, and survey marker stones are frequently buried in humus. You may need to prune branches or rake away fallen leaves. On the other hand, in deciduous broadleaf forests, the ground surface becomes easier to see from autumn through winter. Since light is limited within the forest, it is important to bring a flashlight and carefully check even the darker areas.

Record management to prevent repeated searches at the same site

After locating a reference point, it is important to leave a record to eliminate the need for future searches. This record management is surprisingly often overlooked by field personnel.

When you locate a reference point, document the surrounding conditions with photographs. Take pictures from multiple directions, and be sure to include images that clearly show the positional relationship with identifiable features (utility poles, structures, isolated trees, etc.). Using a smartphone camera with location services, you can also record the coordinates of the photo location. Also record the condition of the reference point (the tilt of the benchmark stone, the degree of wear) so you can track the progression of any damage. When photographing, it is recommended to take multiple shots from different angles, such as a wide-angle shot showing the entire reference point and surrounding features, close-up shots that reveal the benchmark stone and any inscriptions, and shots of nearby landmarks.

Also, by placing marker poles or flags around the reference point, you can make future searches much easier. However, avoid marking the reference point monument itself or altering the surrounding terrain. Reference point monuments are protected by law, and damaging or moving them is strictly prohibited. If you erect marker flags, it is important to secure them firmly so they will not be blown away by strong winds.

Furthermore, sharing information about the control points you located (coordinate values, condition, circumstances at discovery, photos, distances to surrounding features) within the company enhances the overall efficiency of the team. We recommend using a survey management system or a shared cloud to centrally manage this information. When the next person in charge visits the same site, having information such as "this control point is hard to find but is in the brush 15 m (49.2 ft) north of the utility pole" can greatly reduce search time. Especially for companies with multiple branches or offices, building a centralized information management system also helps streamline training of new staff and reduce operational errors.

Standardizing the format of records makes it easier to search for and use information. For example, if you decide on a fixed set of items to record—"reference point name, coordinate values, description of the discovery location, positional relationship with surrounding features, date and time of photography, photographer, and photo file name"—later reference will be smoother.

Utilizing Equipment to Quickly Find Control Points Hidden by Vegetation

High-precision GNSS devices are the equipment that provide the greatest benefit when searching for reference points hidden by vegetation. Because they can provide coordinate guidance with centimeter-level accuracy (cm level accuracy, half-inch accuracy), you can narrow the target location to a radius of several tens of centimeters (several dozen in) and then remove the vegetation. This can greatly reduce unnecessary large-area brush-clearing work. By introducing high-precision GNSS equipment, it is not uncommon for reference point searches that used to take 3–4 hours to be completed in 1–2 hours.

Also, metal detectors are highly effective for locating metal benchmark markers (metal pins and metal markers). Combined with a ground survey after vegetation removal, they can efficiently detect metal benchmarks buried in soil or humus. By becoming proficient in using a metal detector, you can quickly pinpoint the locations of buried benchmarks. When using a metal detector, adjusting the signal sensitivity is important, and care must be taken not to have it respond excessively to nearby metal fragments.

Aerial photography using drones helps to identify vegetation density and terrain features that cannot be determined from the ground. By combining it with pre-existing aerial data, you can make ground search plans more precise. Useful information can be obtained even with small commercial drones. Aerial searches are especially effective for locating reference points over wide areas or at sites where multiple reference points are clustered. Planning ground movement routes in advance from recorded video can reduce aimless wandering on site.

Smartphone-mounted high-precision GNSS devices have lower initial implementation costs than other equipment and are easy to operate, making them easy for small surveying offices to adopt.

An Integrated Approach to Reliably Locating Invisible Reference Points

To efficiently locate invisible reference points, nothing is more useful than high-precision coordinate guidance. By using a high-precision GNSS device that can be attached to a smartphone, RTK positioning enables guidance at centimeter accuracy (cm level accuracy, half-inch accuracy). By leveraging such cutting-edge technology, you can fundamentally and decisively solve the long-standing field problem of searching for reference points.

Even in sites with dense vegetation, if you enter the reference point coordinates into a high-precision GNSS app, you can proceed while checking the direction and distance to the target point on the screen in real time. Because the arrival accuracy is high, there is no need to clear a wide area to search; you can find the reference point simply by checking the ground within a target radius of 30 cm (11.8 in). It also supports solo fieldwork, with a design that allows a single operator to perform grass-cutting and reference point searching simultaneously.

By combining the vegetation countermeasures explained in this article with season-specific measures, terrain-specific strategies, and high-precision GNSS equipment, you can fundamentally solve the field challenge of locating invisible reference points without getting lost. Regardless of the season or terrain at the site, the reassurance that "if you have the coordinates, you can find it" is a great help to field personnel.

High-precision GNSS equipment can be used not only for locating control points at survey sites but also for a wide range of tasks such as stakeout guidance and as-built verification, solving many on-site problems with a single device. The productivity gains from adoption are especially pronounced for companies that frequently work in areas or seasons where locating control points is difficult. For field personnel troubled by invisible control points, please consider using high-precision GNSS equipment. With the right knowledge and appropriate tools, locating control points is not an impossible task but an operation that can be executed reliably and systematically.

By incorporating the content of this article into practical work, you can significantly reduce the time and effort required to locate control points, enabling you to carry out more surveying tasks more efficiently.