Name: Horizontal Control Monument

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Description: Horizontal and Vertical Control (Benchmark) MonumentsThe City of London has installed a network of approximately 400 horizontal control monuments and 600 vertical control monuments (benchmarks) throughout the City to support surveying, engineering, construction, mapping, GIS and facilities management activities. Engineering, surveying and planning Consultants working in the City are required to georeference their work to the City's vertical and horizontal datums described in this document. The supporting horizontal and vertical control monument including location sketches and values are available to the public via the CityMap interactive map accessed through the City's website.Horizontal ControlThe City of London uses the NAD'83 Original datum and 6 degree UTM projection (Zone 17, Central Meridian 81 West Longitude) as its horizontal georeferencing system. Where required, plans submitted to the City for approval must be referenced to this coordinate system. For further information on plan submissions please see Plans and Submission Standards.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 all of the sources are well integrated, the City only supports its own horizontal control network and will not provide values or sketches for other agencies' monuments. The City strongly recommends that only its "028" prefixed monuments be use by consultants.All horizontal control monuments in the City are second order accuracy. 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.Monument location sketches and values can be obtained by clicking Control Map which invokes a special version of the City's interactive CityMap. The monuments will be displayed at a view scale of 5km or larger. To obtain both the location sketch and value of a particular monument, first click on the "Identify" tab at the top of the map and then click on the red triangle (not the text) associated with the desired monument. The location sketch, complete with coordinates, will appear in a separate window for viewing or printing. (You may need to turn off any "popup blockers" in your browser.)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 its number for positive identification (e.g., 028944160). Monuments in grass areas are often marked by 1.5 metre high orange and yellow triangular marker.The elevations on the horizontal control monument sketches are ellipsoid heights that can be useful for GPS surveys. These heights are approximate and are provided to the nearest 0.1 metre only. Ellipsoid heights are not to be confused with geodetic elevations and are not the same as the elevations on the vertical control monuments (benchmarks).Vertical Control (Benchmarks)The City of London's vertical datum is the Canadian Geodetic Vertical Datum 1928 (CGVD28) incorporating the 1978 Southern Ontario Adjustment. Plans containing elevations submitted to the City for approval must be referenced to this datum.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 since they have been integrated properly into the City's network. "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 location sketches and values can be obtained through CityMap by clicking "Control Map" which invokes a special version of the City's interactive CityMap. The benchmarks are displayed at a view scale of 5km or larger. To obtain both the location sketch and elevation of a particular benchmark monument, first click on the "Identify" tab at the top of the map and then click on the monument circle (not the text) associated with the desired monument. The location sketch and geodetic elevation will appear in a separate window for viewing or printing. (You may need to turn off any "popup blockers" in your browser.)The type of benchmark varies depending upon the type. Many 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 horizontally into 4 foot deep concrete structures such as bases for traffic lights and the precise "BM" benchmarks consist of collared bolts drilled into the sides of concrete structures. On BM type benchmark's, the elevation should be taken from the top edge of the hex bolt. For further informationIf you have any questions or wish to report damaged or destroyed monuments please contact Geomatics Division at 519 661-2500 Ext. 4908 (phone) or 519 661-6422 (fax) or by emailing Geomatics.

Copyright Text: Corporation of the City of London | Produced by: Environmental & Engineering Services - Roads & Transportation/ Geomatics

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