Demystifying Surface Elevation Measurements in Mines

Geologists should have a strong understanding of how elevations and contours are measured in a mine. This knowledge is crucial for accurate topographic analysis, mine planning, and mineral reserve estimation. Geologists need to be familiar with various surveying techniques, including differential GPS (dGPS), total station surveys, and drone-based LiDAR surveys, which are commonly used to determine elevations and contours in a mine. Additionally, understanding the reference systems and geoid models used to convert elevation data is essential for proper elevation calculations. Geologists should also be aware of the importance of establishing permanent benchmarks for maintaining consistent elevation references over time.

If elevation calculations are off by just one meter in a mine covering one hectare, the resulting discrepancy can lead to a difference of 10,000 cubic meters (equivalent to 25,000 metric tons) in the computed reserve of granite building stone. Such an error could ultimately result in a loss of Rs. 12,00,000 in royalty payments. Suppose such mine is working illegally, then it could result in a loss of Rs. 60,00,000 in terms of both royalty and the value of the mineral.

The contour map of the proposed area is a critical prerequisite for granting quarrying permits or leases. Geologists and Recongnised Qualified Persons (RQPs) employ various methods to determine the terrain elevations. Unfortunately, mineral administrators often overlook the specific survey techniques used by RQPs. Some RQPs utilize differential GPS (dGPS), while others rely on handheld GPS devices to establish latitude, longitude, and elevation at boundary points. Alternatively, some RQPs extract elevation data from Google Earth, while others refer to Survey of India topographic sheets for contour map preparation. Additionally, certain RQPs establish temporary benchmarks for carrying out  contour mapping.

The situation becomes more intricate when a mining scheme is prepared by different Recognised Qualified  Persons who employ entirely different methods to determine elevations. To gain a deeper understanding of topography, one must familiarize themselves with various survey techniques and the different types of elevations or heights obtained through sources such as total station surveys, differential GPS (dGPS) surveys, handheld GPS surveys, and other references like topographic sheets, Google Map elevations, and SRTM data.

Elevation using GPS receiver:

It is very important to note that GPS receivers measure elevation with respect to ‘the ellipsoid”.

What is an ellipsoid?

All elevation measurements are, in essence, the difference between the earth’s surface and a point above or below it. But because the earth’s surface is full of physical anomalies and is changing constantly, scientists rely on theoretical representations — called “vertical datums” — to represent the earth’s surface.

An “ellipsoid” is a 3D shape like a sphere, but more like an oval. The earth is shaped like this, with the north and south poles acting as the top and bottom points of an approximate egg. The ellipsoid is a mathematical conception of the earth’s surface, which is referenced by GPS / GNSS receivers because it is extremely accurate. So, when a receiver collects elevation data, it is referenced to the ellipsoid.

However, there is a problem with ellipsoidal elevations. Although they are very accurate, they are not practical for every day operations, such as field work.

The issue is that the earth is not a perfect ellipsoid. It has mountains, craters, and other features above or below the mathematically perfect ellipsoidal reference. Therefore, GIS users must transform their ellipsoidal data into a practical elevation reference.

Understanding Ellipsoidal Height: Key Considerations:

·         All elevation data is based on a defined “vertical datum”

·         A vertical datum is a representation of the earth’s surface (considered to be 0 meters)

·         GPS receivers measure elevation with reference to an ellipsoid

·         Ellipsoidal data is accurate, but not a common elevation reference

 

The Mean Sea Level

The vertical reference often used to represent the earth’s surface called the “mean sea level,” or MSL. MSL is a local tidal datum that can be used as a reference for elevation when close to the shoreline. However, once you get more than a few kilometers inland, MSL becomes impractical.  Your GPS / GNSS receiver already outputs global MSL, because MSL is a standard for position output.

MSL can be calculated two ways. First, it can be measured locally, by taking data over time about the highs and lows of ocean tides (based on the gravity of the moon, sun, earth, and other variables). And by averaging the past 19 years of these global sea highs and lows, scientists have also been able to create a global MSL, which GPS receivers sometimes used as a reference for measuring elevation.

However, it’s important to note that the global MSL on your GPS receiver is generally based on a coarse 10-minute by 10-minute grid. This can make the global MSL elevations output by GPS receivers off by several meters.

So how can you use your GPS receiver elevations for practical applications?

Important points:

·         Mean sea level “MSL” is an elevation reference output by GPS receivers

·         MSL elevations don’t take into account local factors

Geoid Model

The geoid is similar to, and sometimes confused with the MSL because both are based on similar factors (such as gravitational forces). However, the geoid is far more accurate because it is a locally calculated geometric representation of the actual physical shape of the earth.

 A geoid model is a location-based grid that allows you to convert between ellipsoid and  a national vertical datum such as NAVD88. Geoid models are country-specific. 

The geoid model contains an offset value called the “geoid height” or “geoid undulation.” The geoid height is a locally specific, constant number that represents the vertical difference between the reference ellipsoid and the geoid in that area. The geoid height can be positive or negative. This is the number we must use to convert between ellipsoidal and local vertical datum heights. These elevations are called the orthometric heights.

Orthometric height is the type of elevation data your surveyors, engineers, and other field workers need to work practically and accurately.

 Ellipsoid to Geoid conversion

Because geoid height is a mathematical offset between the ellipsoid and the geoid, it can be used to translate elevation data from one vertical reference to another. This means the geoid height provides the key to unlocking your GPS receiver’s accurate elevation data.

For most surveyors, GIS users, and others, they will want to convert the ellipsoidal data into an elevation measurement called the “orthometric height.”

Important points:

·         The vertical datum is an accurate physical representation of the earth’s surface

·         A geoid model is a locally defined grid that allows conversion from ellipsoidal to orthometric heights

·         A geoid height is a number within the geoid model that enables this conversion

·         The orthometric height is the practical elevation that tries to describe the heights of points on the earth’s surface

Calculating orthometric height from ellipsoidal data with the geoid height

How do we use geoid height to calculate orthometric elevation? We’ll use this formula:

H = h – N

The three variables represent terms we’ve already defined. Here is what they stand for and where they come from:

Variable:	Represents:	What it is:
H	Orthometric Height	This is the elevation our surveyors and field workers need
h	Ellipsoidal Height	This is the elevation above or below the reference ellipsoid from our GPS receiver
N	Geoid Height / Undulation	This is the offset between the geoid and ellipsoid references; we find N in the geoid model used

Important points

·         The formula for calculating orthometric height is “H = h – N”

·         You need the geoid and ellipsoidal heights to perform this conversion

 

Understanding Elevations from Different Sources: Toposheets vs. Google Maps”

Toposheet Elevation:

The Survey of India’s toposheets provide elevation information in meters above or below mean sea level (MSL).

The MSL reference adopted by the Survey of India is based on Bombay height (Mumbai).

It’s important to note that mean sea level can vary along different coasts. For instance, there is a 30 cm difference in MSL between Mumbai and Vishakapatnam.

Contour values printed on toposheets represent elevations relative to mean sea level.

Google Map Elevation Data:

Many people now rely on Google Map elevation data. In Google Earth, elevation is displayed when you hover the mouse over a location. However, Google does not disclose the specific source of this elevation data. It is presumed that Google uses various digital elevation models (DEMs) generated from different sources. A closer examination of data reveals that in Kerala, there is a consistent difference of approximately 19 to 20 meters between elevation points on toposheets and Google Maps.

SRTM elevation data:

The Shuttle Radar Topography Mission (SRTM) collected elevation data on a near-global scale, resulting in the most comprehensive high-resolution digital topographic database of Earth. SRTM utilized a specially modified radar system aboard the Space Shuttle Endeavour during an 11-day mission in February 2000. This international project was jointly led by the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA).

Key points about SRTM data include:

1. Resolution:
• The highest-quality SRTM data has a resolution of 30 meters. This means that each elevation reading corresponds to a 30x30 meter area of land.
• Consequently, changes in elevation within a 30-meter radius are not captured by SRTM data.
2. Temporal Considerations:
• SRTM data specifically represents conditions as of the year 2000.
• If a mine was operational before 2000, the pits from that period will be accurately reflected in the SRTM data.
• However, any pits formed after 2000 will not be visible in the SRTM dataset.
3. Geoid Reference:
• SRTM data benefits from using the EGM96 geoid model, which closely approximates mean sea level elevations depicted in the Survey of India’s toposheets.

In summary, while SRTM provides valuable elevation information, users should be aware of its limitations and consider other data sources for more recent or localized details.

GPS elevation data:

Elevations collected during dGPS surveys are typically provided as ellipsoidal heights. However, dGPS can also yield orthometric heights by referencing a specific geoid model. In India, the commonly used geoid model for GNSS (Global Navigation Satellite System) applications is IndGG-SH2021. It’s crucial to select the appropriate geoid model to ensure accurate height determination in GNSS applications, particularly when converting ellipsoidal heights to meaningful orthometric heights.

Setting up of permanent bench mark with arbitrary elevation

A benchmark serves as a fixed, precisely known reference point used in surveying to establish and maintain height (elevation) measurements. These benchmarks play a crucial role in ensuring consistent vertical references across various surveys and projects. When conducting topographic surveys, geospatial professionals often begin by fixing a known point as a benchmark. For instance, assigning an elevation of 100 meters above mean sea level (MSL) to a benchmark provides a reliable starting point. Using this benchmark, elevation differences across the terrain can be measured, leading to the creation of contour maps that are valuable for calculating mineral reserves.

To maintain consistency, a durable structure (often made of concrete) can be established at the benchmark location. This structure includes latitude, longitude, and an arbitrary elevation etched /painted onto it. The coordinate and elevation details of this benchmark must be documented in the mining plan for future reference. Additionally, assigning the benchmark’s elevation using Shuttle Radar Topography Mission (SRTM) data ensures reliability and consistency. SRTM provides global elevation data with a resolution of 30 meters, allowing users to verify the accuracy of their benchmarks and make informed decisions in various applications. 

Drone-based LiDAR (Light Detection and Ranging) surveys offer a powerful method for capturing elevation data across the Earth’s surface. Widely used in mine surveys, this technology provides precise measurements for each point on the ground. Here are the key considerations:

LiDAR-equipped drones emit laser pulses and measure the time it takes for the reflected signal to return. By analyzing these return times, LiDAR creates detailed 3D point clouds representing the terrain. To ensure accuracy, ground control points (GCPs) are essential. These fixed reference points have accurately known coordinates (both horizontal x-y and vertical z). GCPs help align LiDAR data with real-world coordinates, improving the precision of elevation measurements. LiDAR surveys benefit mine planning, volumetric calculations, and environmental assessments. Establishing a consistent benchmark or permanent GCP ensures uniform elevation references. Regular maintenance of these benchmarks is crucial throughout the mine’s lifespan.

In summary, drone-based LiDAR surveys offer unparalleled accuracy for terrain mapping, making them indispensable tools in modern mining operations. 

 Summary

The assessment of mine elevations is a legal requirement before a mine begins operations and at five-year intervals thereafter, as stipulated by mineral rules. Documents such as mining plans, scheme of mining, and final mine closure plans include contour maps representing the mine’s topography at various stages. Geologists heavily rely on these contour maps and spot heights to calculate the volume of minerals extracted. However, a significant challenge arises due to varying base elevation values used by different Resource Quality Planners (RQPs).

Consider the following best practices for mine elevation measurement:

 

1. dGPS Survey with Accurate Geoid Model:
• Conduct a differential GPS (dGPS) survey, ensuring accurate geoid modelling (IndGG-SH2021).
• The dGPS-based coordinates and elevation values of boundary pillars should be precisely fixed and marked.
2. Permanent Benchmark Establishment:
• Erect a permanent benchmark outside the mining area.
• This benchmark should have well-defined coordinates and a meticulously computed elevation value.
• Throughout the mine’s lifespan, maintain this benchmark to ensure consistent elevation references.
3. Total Station Survey for Further Measurements:
• For subsequent measurements (such as scheme of mining or final mine closure plans), use the permanent benchmark as the reference level.
• Total station surveys suffice for these purposes, maintaining consistency across all assessments.
4. Alternative Approach without dGPS:
• If dGPS is unavailable, consider the following:
• Identify a known point (preferably with similar elevation characteristics) within a 30 m x 30 m area.
• Obtain the Shuttle Radar Topography Mission (SRTM) elevation value for this point.
• Establish a permanent benchmark and conduct total station surveys to determine area levels relative to this benchmark.

By adopting standardized procedures and leveraging available technologies, geologists can achieve reliable and consistent elevation assessments for mining operations.

Recommendations:

1.    Before scrutinizing the plans and sections of a mine, the geologist should be aware of the methods used for the survey as well as the way elevation is taken. There shall be specific mention in mining plan regarding the type of survey carried out, equipment used, the how the elevation values are obtained and the level of accuracy.

2.    The geologist, while visiting the mine site for inspection of mining plan, may instruct the mine owner to fix one permanent bench mark with latitude, longitude and elevation written on the same based on the survey conducted for the preparation of plans and sections. The bench mark shall be fixed outside lease area.

3.    While processing the scheme of mining, care should be taken to ensure that the elevations have a common reference with respect to the benchmark established.