Using Google Satellite & Open Street Maps as the Base for Copan Ruinas GIS System

 Using Google Satellite & Open Street Maps as the Base for Copan Ruinas GIS System

Background

We are all familiar with using Google Map for location and navigation with our mobile phones. We can see our track as we drive along the roads on the map. It generally falls within the width of the road, especially if we are zoomed out. Similarly we can see where we are on the map if we are looking for a restaurant or store in town. The single band GPS/GNSS (L1) receiver in our phone gives us a location within 5 to 10 m in urban areas, normally that is enough for what we need. In addition the Google satellite image also is accurate within 2 to 15 m depending on where you are in the world.

We can see that when we are using it while we are walking along a street. Sometimes if we look at the image it shows that we are on the right side of the road even though we are walking along the left. This is because of the accuracies of the GNSS receiver and the google map image registration. If we need a more accurate location then we should use survey grade dual band GNSS receivers (L1/L2 or L1/L5) and a properly georeferenced satellite image.

Ultimately I would recommend that Copan Ruinas use precise coordinates and a georeferenced satellite image for its GIS system but that is not possible at this time. As a result of my assignment in Copan I am recommending that the Google Satellite image and Open Street Map (OSM) be used as the base map for adding Copan's GIS layers.

For example one GIS layer could be the water line network in the urban areas of town. The original as-built survey was undertaken in 2006 using benchmarks listed below for the survey. It appears that the NAD27 coordinates used for the benchmarks need to be translated to make the network line up with the actual NAD27 coordinates shown in Google Map. 

Even after translation the as built drawings for the water or sewer manholes, valves, etc. could be a few meters off (because Google Map is not precisely georeferenced). However it will be close enough to find the location of manholes, etc. with a Garmin or mobile phone. It should also show the water line connecting to the right property most of the time if the cadastral base map also uses the same system.

It should be noted that the water, sewer and cadastral maps in Copan uses the NAD27 UTM Zone 16N coordinate reference system (CRS).

Water Supply System

Public Works provided the Water Supply as-built drawing from AutoCAD. This was imported into QGIS and exported to a shape file. The CRS in QGIS is set to NAD27 UTM Zone 16N as shown below.


As you can see it does not line up with the Google image. However if we used the QGIS > Processing Option > Translate plugin and we input DX = -13.41, DY = -99.85 m respectively we get the following result shown below.


Now the as-built water line lines up with the roads in town using OSM. The same translation can be done in AutoCAD.

Sewage Collection System

Similarly the sewage network as-built used the same benchmarks for their total station survey so the same translation offset can be used.


The following lists the displacement or translation to match Google Map. The DX and DY values could be fine tuned if necessary.

Copan Survey Benchmarks

The following were the benchmarks used for the as-built surveys of the water, sewer and water lines to the intakes. The drawings are available from Public Works.

It should be noted that benchmark U 105-1 is the benchmark in front of the Copan Ruinas town hall. The values of the benchmarks do not match existing NAD27 values. This could be because someone in 2006 used a value supplied by others. I am still seeking what coordinate value the national government has assigned to U 105-1.

Cadastral Map

The Cadastral Department uses handheld single band L1 GPS/GNSS receivers for their field surveys. This equipment provides predictable positioning accuracy of 9 m (95%) horizontally and 15 m (95%) vertically. With averging and WAAS this could be improved to 2 to 5 m. Regardless this poses a problem when you are trying to join lots to make up a subdivision. Errors could compound.


Importing the Mapas Urbano y Rurral Copan Ruinas 2023 Modificado 14 into Google map with QGIS the lots and blocks do not fit very well. Unlike the as-built surveys there is no DX, DY translation that would work for the whole town.

It is my opinion that the best option was to translate each block to the block shown on the OSM map (or Google Map). In addition to translation, in some instances you would need to rotate the block as well to make it fit. The steps in QGIS is to use is the editing function in the Advanced Digitizing Toolbar.  Select the shapefile > Edit on > Select the block > Move feature. Click on the selected objects and use your cursor to move the selection to where you want. If you need to rotate then select the rotate option.


The completed move, rotate, etc. is added and shown below.



The above shows that it is possible to create a good registration to Google and OSM with the cadaster and Public Work services like water and sewer on the same base map. If a better registration is availble it would be easy to translate the whole system to match the accurate locational data.

A note for the Cadatral Department is the coordinates for real property is based on the horizontal projection (orthographic projection) of land boundaries, meaning they show the area as if it were a flat, level surface, regardless of the actual slope, topography, or elevation of the site. So this needs to be taken into account when you measure the sides of a lot on a sloped property otherwise errors will compound.


Comments welcome.






Comments

Post a Comment

Popular posts from this blog

GPS Surveying & Drone Mapping for Public Works and Cadaster Departments

Image
  Background Historically local governments in many countries used optical survey instruments to map their cadastre and public works projects. Many municipalities establish a baseline using a survey control benchmark in the square near their municipal offices to establish a baseline to measure their surveys from.  In the Philippines the U.S. Army Corps of Engineers originally established the geographic coordinates of these control monuments using celestial observations. These coordinates have been updated with the launching of the GPS satellites by the U.S. Department of Defence in 1978.  Since then Europe, China, Russia and others have added their own constellations of satellites to augment positional accuracy and to decrease the reliance on one nation for that service. They are referred to the Global Navigation Satellite System (GNSS) as a whole for precise geo-spatial positioning, navigation and timing (PNT) services. They are named respectively Galileo, BeiDou, Glonas...
Read more

Solid Waste Stockpile Monitoring with Drone Photogrammetry on a Budget

Image
Solid Waste Stockpile Monitoring with Drone Photogrammetry on a Budget Background Solid waste stockpile monitoring involves the systematic, real-time, or periodic evaluation of waste piles to manage volume, ensure environmental safety, and maintain regulatory compliance. Modern monitoring techniques, such as 3D LiDAR scanners and UAV (drone) photogrammetry, have largely replaced manual, less accurate estimations, allowing for safe, continuous, and high-resolution tracking of waste quantities.  The following is a step-by-step example to show how local government with limited resources and equipment can monitor and manage their solid waste sites using low cost consumer drones like the DJI mini 4 Pro and single phase GPS/GNSS hand held receivers for ground control with Open Source applications like WebODM. Naturally better results can be achieved with higher grade equipment but I will show in this web blog that anyone can start even with limited resources.  The Basic Tools and Eq...
Read more