3 ms·

Laughable because LA City's system, Zimas, has property lines so poorly drawn over their satellite / aerial imagery. It is some simple alignment issue that puts

by carom 4y ago

Laughable because LA City's system, Zimas, has property lines so poorly drawn over their satellite / aerial imagery. It is some simple alignment issue that puts your property line in your neighbors driveway, or your neighbors easement fully on their property. The sad thing is, this translates into surveys. Now my neighbors and I have completely nonsensical stamps in the ground that we have to figure out how to correct. Sad thing is, it lines up in their 2001 imagery, but 2017 is off by a few feet.

defrost 4y ago
> The sad thing is, this translates into surveys. Now my neighbors and I have completely nonsensical stamps in the ground that we have to figure out how to correct. Are these "stamps in the grounds" actual survey pegs placed by licenced surveyors working from the official Lands & survey database after having triangulated from previously placed officvial survey pegs? Misalignment of geomosaiced air photo images | poor rectification is one thing (and not altogether uncommon) but that's a world away from ground based triangulated surveying backed by dual GPS against ground station modern survey techniques (which extend from a time before air photo surveys became common).
- wyclif 4y ago
  Ex-land surveyor here. They're called corner monuments or property corners. However, in residential areas they are usually not concrete monuments but rather a piece of iron rebar: https://www.cardinalsurveying.com/property-corner-markers/ https://www.cardinalsurveying.com/property-corner-markers/
  - defrost 4y ago
    I'm familiar with monuments | pegs, I'm hoping the commenter clarifies their statement about "stamps" on their property. I'm not clear whether their complaint is entirely about airphotos and GIS lines not being rectified (annoying but not critical) or if they're actually claiming that visual misalignment of two seperate data sets has caused monuments to have been incorrectly placed. Here (W.Australia) that wouldn't happen. FWiW many years back I contributed code to software in this field ( https://download.hexagongeospatial.com/en/downloads/imagine/erdas-er-mapper-2022 https://download.hexagongeospatial.com/en/downloads/imagine/... )
- s1artibartfast 4y ago
  IT sounds like someone skipped a step in the data tracability and they are now putting survey pegs in the wrong position based on bad GPS coordinates.
  - defrost 4y ago
    To be honest it just sounds fishy. The "lines on the (digital) maps" are generally sourced from official public records and have the correct GPS (or local grid) coords. The images themselves originate from digital capture behind lenses and are subject to subtle stretching and distortion in addition to positional errors in camera position. It's common to see major visual parts of any "corrected" frame line up exactly with ground reference coords but other parts of the same frame to off by a few metres due to rough and ready "mostly linear" correction that doesn't account well for the digital terrain model the image must draped across (in addition to lens corrections). Point being - while digital maps can often have alignment errors twixt image pixels and GIS vectors the actual on the ground land surveying by professionals is done using high grade GPS units in addition to truthing from existing monuments using triangulation. Staking pegs based on looking at a digital map isn't something a professional would normally do as they'd be aware of the various errors (and others) I raised.
- carom 4y ago
  I believe they are official ones. Little metal circles pinned into the cement. Survey was done by the previous owners. They line up with the ortho imagery and not with the logical property lines (what the entire street and every property on it are aligned to). The ortho imagery has the front of the properties too far back on one side of the street and too far forward on the other, so it is not a single property line issue. The data set [1] used has tolerances listed but it is frustrating that it translates to a survey. Some examples of not my house. [2] 1. https://lariac-lacounty.hub.arcgis.com/pages/lariac5-documents-data https://lariac-lacounty.hub.arcgis.com/pages/lariac5-documen... 2. https://imgur.com/a/Nqkdkjj https://imgur.com/a/Nqkdkjj
  - empyrrhicist 4y ago
    Sounds like the people who built the neighborhood may have screwed up? Otherwise it's common for things like driveway replacements to accidentally (or purposefully) overextend when the real property lines are forgotten.
  - defrost 4y ago
    The metal circles you describe are likely the official survey points and likely marked well after WGS84 World Geodectic datum | universal GPS coords .. in a neighbourhood that may have been first marked out sometime before (?) (Perhaps the 70s | 60s | 50s ? - I'm from Australia and not overly familiar with time + architectural style from air photos in your area). The photo image accuracy in your first link is quoted as: * 1.35 feet at 95% confidence interval * 3.63 ft at 95% confidence for two seperate products .. you can read that as most displacements (of pixels (say a footpath corner)) should be less than either 1.5 or 2.5 feet (depending on product) but you should recall that some parts of a larger mosaic will have areas with larger errors and other parts will be damn near spot on. If the "modern" GPS era monuments | pegs | nailed discs in concrete don't precisely align with the (I assume) pre GPS era road edges, foot paths, easements, etc then that can be put down to initial developer first survey doing a job that was "good enough for the 70s" (or the era of the time). These days many land councils focus on the "total gestalt" of a series of co developed blocks - are the individual block all of the correct dimensions and areas .. (even if shifted by a foot or two from the "expected") ? This is an extremely common issue that arose right around the globe as GPS became the global standard and many (many many) previously seperate datums and local area grids were tied together in single unified system.

by carom 4y ago

defrost 4y ago

> The sad thing is, this translates into surveys. Now my neighbors and I have completely nonsensical stamps in the ground that we have to figure out how to correct. Are these "stamps in the grounds" actual survey pegs placed by licenced surveyors working from the official Lands & survey database after having triangulated from previously placed officvial survey pegs? Misalignment of geomosaiced air photo images | poor rectification is one thing (and not altogether uncommon) but that's a world away from ground based triangulated surveying backed by dual GPS against ground station modern survey techniques (which extend from a time before air photo surveys became common).

wyclif 4y ago
Ex-land surveyor here. They're called corner monuments or property corners. However, in residential areas they are usually not concrete monuments but rather a piece of iron rebar: https://www.cardinalsurveying.com/property-corner-markers/ https://www.cardinalsurveying.com/property-corner-markers/
- defrost 4y ago
  I'm familiar with monuments | pegs, I'm hoping the commenter clarifies their statement about "stamps" on their property. I'm not clear whether their complaint is entirely about airphotos and GIS lines not being rectified (annoying but not critical) or if they're actually claiming that visual misalignment of two seperate data sets has caused monuments to have been incorrectly placed. Here (W.Australia) that wouldn't happen. FWiW many years back I contributed code to software in this field ( https://download.hexagongeospatial.com/en/downloads/imagine/erdas-er-mapper-2022 https://download.hexagongeospatial.com/en/downloads/imagine/... )
s1artibartfast 4y ago
IT sounds like someone skipped a step in the data tracability and they are now putting survey pegs in the wrong position based on bad GPS coordinates.
- defrost 4y ago
  To be honest it just sounds fishy. The "lines on the (digital) maps" are generally sourced from official public records and have the correct GPS (or local grid) coords. The images themselves originate from digital capture behind lenses and are subject to subtle stretching and distortion in addition to positional errors in camera position. It's common to see major visual parts of any "corrected" frame line up exactly with ground reference coords but other parts of the same frame to off by a few metres due to rough and ready "mostly linear" correction that doesn't account well for the digital terrain model the image must draped across (in addition to lens corrections). Point being - while digital maps can often have alignment errors twixt image pixels and GIS vectors the actual on the ground land surveying by professionals is done using high grade GPS units in addition to truthing from existing monuments using triangulation. Staking pegs based on looking at a digital map isn't something a professional would normally do as they'd be aware of the various errors (and others) I raised.
carom 4y ago
I believe they are official ones. Little metal circles pinned into the cement. Survey was done by the previous owners. They line up with the ortho imagery and not with the logical property lines (what the entire street and every property on it are aligned to). The ortho imagery has the front of the properties too far back on one side of the street and too far forward on the other, so it is not a single property line issue. The data set [1] used has tolerances listed but it is frustrating that it translates to a survey. Some examples of not my house. [2] 1. https://lariac-lacounty.hub.arcgis.com/pages/lariac5-documents-data https://lariac-lacounty.hub.arcgis.com/pages/lariac5-documen... 2. https://imgur.com/a/Nqkdkjj https://imgur.com/a/Nqkdkjj
- empyrrhicist 4y ago
  Sounds like the people who built the neighborhood may have screwed up? Otherwise it's common for things like driveway replacements to accidentally (or purposefully) overextend when the real property lines are forgotten.
- defrost 4y ago
  The metal circles you describe are likely the official survey points and likely marked well after WGS84 World Geodectic datum | universal GPS coords .. in a neighbourhood that may have been first marked out sometime before (?) (Perhaps the 70s | 60s | 50s ? - I'm from Australia and not overly familiar with time + architectural style from air photos in your area). The photo image accuracy in your first link is quoted as: * 1.35 feet at 95% confidence interval * 3.63 ft at 95% confidence for two seperate products .. you can read that as most displacements (of pixels (say a footpath corner)) should be less than either 1.5 or 2.5 feet (depending on product) but you should recall that some parts of a larger mosaic will have areas with larger errors and other parts will be damn near spot on. If the "modern" GPS era monuments | pegs | nailed discs in concrete don't precisely align with the (I assume) pre GPS era road edges, foot paths, easements, etc then that can be put down to initial developer first survey doing a job that was "good enough for the 70s" (or the era of the time). These days many land councils focus on the "total gestalt" of a series of co developed blocks - are the individual block all of the correct dimensions and areas .. (even if shifted by a foot or two from the "expected") ? This is an extremely common issue that arose right around the globe as GPS became the global standard and many (many many) previously seperate datums and local area grids were tied together in single unified system.