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Georeferencing and Survey Control Monuments


 

 

 

Location Sketches and Values

Monument and benchmark location sketches and their associated values are obtained from the interactive control monument map.

Monument and benchmark locations and labels will be displayed once you zoom into the area of interest. To view the location sketch and coordinate value assocated with a particular monument simply click on the centre of the monument symbol, not the label. The location sketch will be generated in a separate window for viewing or printing. Note that It may be necessary to turn off popup blockers in your browser in order to view the location sketch.

Georeferencing

The City of London uses the NAD'83 (Original) datum, 6 degree UTM projection (Zone 17, Central Meridian 81° West Longitude) as it's horizontal georeferencing system, and the Canadian Geodetic Vertical Datum 1928 (CGVD28) incorporating the 1978 Southern Ontario Adjustment as it's vertical georeferencing system. All legal surveys, engineering drawings and plans submitted for development approvals including subdivisions and site plans must be referenced to these datums. This enables the City to efficiently assess, review and integrate the information into its GIS datasets, and ensures information can be more easily exchanged with others. For more detailed information please see Plans and Submission Standards.

In support of this georeferencing requirement, the City has installed a network of approximately 400 horizontal control monuments and 600 benchmarks throughout the City.

Horizontal Control Monuments

There are a number of sources for horizontal control monuments in the City of London including Natural Resources Canada (NRCan), Ministry of Natural Resources Ontario (MNR), Ministry of Transportation of Ontario (MTO), Transport Canada, and the City of London. Note that not all City of London monuments are registered in the Provincial COSINE database. Although the monument values from all sources are compatible with each other, the City only provides sketches and values for it's own "028" prefixed monuments.

All "028" horizontal control monuments in the City are second order monuments. In simple terms, the relative horizontal accuracy between any two monuments is about 1-2 cm, or 0.0005 arc-seconds of longitude or latitude.

Horizontal control monuments consist of either bronze caps on a 1.5m x 0.025m diameter iron bar buried 0.2 metres below the surface of the ground, or a bronze cap cemented into a concrete sidewalk or curb. Each monument is stamped with a unique number for positive identification (e.g. 028944160). Monuments in grass areas may be marked by 1.5 metre high orange and yellow triangular marker.

Note the elevations shown on the horizontal control monument sketches are ellipsoid heights and are not be be used to establish elevations. 

Vertical Control Datum and Benchmarks

There are many benchmarks in the City from various sources including Natural Resources Canada, Ministry of Natural Resources (Ontario), Ministry of Transportation of Ontario, Transport Canada, and the City of London. Although most are well integrated with the City's vertical control network, Transport Canada Benchmarks near the London International Airport must not be used since their values are not compatible with the 1978 Southern Ontario Adjustment.

Wherever possible, the City's "BM" or "V-type" benchmarks should be used. "BM" series benchmarks are first order accuracy and "V" series benchmarks are third order. The misclosure between BM series benchmarks should be less than 0.004m x SQRT(km) and less than 0.024m x SQRT(km) for all other benchmarks.

Benchmark monuments come in many forms. Some consist of bronze caps on 1.5m x 0.025m iron bars or bronze caps cemented vertically into concrete foundations. The City's V-type benchmarks usually consist of 1/4" x 2" "nail-in" concrete anchors drilled vertically on top of concrete structures such as bases for traffic lights. Precise "BM" benchmarks consist of collared stainless steel bolts cemented horizontally into the sides of concrete structures. For "BM" type benchmark's, the elevation should be taken from the top-most vertex of the hex bolt.

Reporting Destroyed Monuments

Please report any damaged or destroyed monuments to Geomatics.

 

 

 

 

 

 

 

 

 

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