Sub-Meter Map Relativity

General Discussion How To
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There may be times when you want to have your maps align and be able to report progress, but don’t need centimeter absolute accuracy or have GNSS equipment to set GCP’s. This is a process I use for these occasions. You can paint the ground, put down some inexpensive targets or just use existing features to create rough Ground Control Points or RGCP’s as we call them. With the aid of Google Earth Pro, QGIS or other GIS software you can create a control file to upload with your images that will allow you to compare your maps side-by-side perfectly aligned, have persistent overlays and even calculate work performed between maps.

Now here is the disclaimer. This does not tie your maps to any surface (localized) coordinate systems and is a sub-meter alignment with WGS84 which can then be transformed to an approximation of another CRS via DroneDeploy or GIS software like QGIS. You can however use the proper GNSS equipment to localize this control to create a network by which you can assist with mass land development. An example would be having a private land owner who might have the need to rough grade a site, build a barn or build a pond ON THEIR PROPERTY. This is not applicable to commercial work that would require permitting. It can however be applied in conjunction with an Engineer to provide design development and budgeting numbers. You could use your control network to design and layout features that are relative to that network within a tenth of a foot.

Here’s the workflow,

  1. Set some inexpensive targets (Home Depot 5-gallon bucket lids) that can be left on site or set markers that the targets can be returned to on future visits. If you do not have access to the site then you can later pick existing features such as corners of structures or paving, manholes, inlets, fence corners or any other feature that is unlikely to be disturbed that you can see in your drone map.
  2. Fly your drone.
  3. Process the map without GCP’s.
  4. Load the map into Google Earth or QGIS.
  5. Use the drone surface elevations.

Processed Map to GCP’s
Google Earth
QGIS (more accurate)

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Why did u process without GCP to later in qgis spot it?dont understand

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Creating gcps off of the first map instead of prelocated gcps is the easiest way to make relative maps that will align to each other and can be used for progress tracking. Having prelocated gcps even off of a phone will be more accurate than the map processed without gcps.

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Is it possible to know how accurate is my map?

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This method will get you to 2-3 meter absolute global accuracy, but 5cm relative accuracy between maps. You would have to have at least two known elevation reference points preferably on opposite sides of the site as checkpoints.

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Thank you guys for the feedback.

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I never did ask, but what do you think the relative accuracy would be? Vertical - Horizontal?
Not in relation to the rest of the world i.e. absolute accuracy. I am not looking for maps to align just merely give me fairly accurate elevation/contour maps.

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If I just fly with no gcps or doing the other stuff you suggested, not that I might not try that…what would you guestimate relative accuracy to be?

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The main point of this workflow is to get repeatable horizontal so that you can compare maps side-by-side and we are seeing 5cm but the vertical is mostly dependent on the GPS coverage and the environmental conditions like the wind in which we are seeing as good as 0.5m and worst at 1.5m. If the volumes on things like stockpiles is the main goal then it is plenty good unless the stockpiles cover more than an acre. If you are trying to get volumes of the change of the whole site then I don’t think this will work and you will need actual GCP’s.

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Thank you again. I will discuss with my client to find out what their expectation is. So as not to give them something that is useless to them. I will find out more of their needs and go from there.

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