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Doubt about my geo referencing procedure

KenLucas

I just completed creating 8 links to rectify a 1948 aerial photo TIFF to 2013 NAIP imagery, using Arc 10.2 I'm skeptical of the Residual (RMS) error, which I use to judge how effective the links are. Please see the attached screen capture. I believe that the RMS error is in the units of the arc map session, in this case, Meters. Specifically, I'm skeptical of the 0000884777 Total RMS error. Does this look reasonable? I'm linking points on the 1948 TIFF to the same points that I've ID'd on the 2013 NAIP. Should I be linking: TIFF to Imagery? Alternatively, should I create some vector point features, overlaid on the 2013 NAIP, and then link: TIFF to point features?
Ken

Capture1.JPG

Amanda__Huber_0-1743107549502.png

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Accepted answers

MelitaKennedy

Hi Ken,

My rough approximation of degrees-to-meters is to say that the 4th decimal place is about 30m, so you're seeing a fit around 3 m or less.

If you can, it's sometimes better to georeference directly to the native coordinate system of the raster. Then reproject it if necessary. It's more necessary as scale gets smaller (larger area is represented).

Melita

All comments

MelitaKennedy

RMS is in the units of the coordinate system which is, in this case, decimal degrees (your links are lat/lon).

Melita

KenLucas

Melita,
Thank you for responding. I realize the X & Y figures in the link table are lat & long. My doubt is about the total RMS error. Is that a reasonable RMS error to you (meters, in the map units)? I just tried creating point features on the NAIP imagery and then linked the 1948 tiff to those. 7 of these links resulted in a higher RMS error, although still well below 0.0 In geo referencing I've done before it required a lot of links to get just to a 0 RMS; course that work was with GIS 9.3
Ken

MelitaKennedy

Perhaps I'm not understanding something. It's been a while since I georeferenced data. The total RMS values are in decimal degrees, because that's what the target links are in. If you want RMS in meters, you're going to have georeference to a map that's in a projected coordinate system or use links with target values in meters.

Melita

KenLucas

My original questions remain un answered. Is the RMS error, in the original attachment, realistic; does it tell me anything? Is it right to link pts in a tiff directly to a image raster? Can someone suggest how I might optimize the settings in the attached capture Rectify window? Please see attached.
Ken

Capture2.JPG

MelitaKennedy

KenLucas

Melita,
Your last note was key to helping me understand how I need to think of the RMS in terms of degrees error. I've tried, unsuccessfully, to reverse engineer your approx. math, where you came up with 3 meters of error: degrees to meters. Would you please tell me what is wrong with my conversion math:

1 degree = 111 kilometers. To convert our RMS error: (0.0000884777 * 111) * 1000m = 9.8 meters of error

Instead of re projecting the imagery, to gain more accuracy, can we not just change the arcmap session to the native projection, of the imagery, to eliminate the error as ArcMap projects the imagery on the fly?
Ken

MelitaKennedy

My conversion comes from:

6378137 * 2 * PI / 360 = 111.319 km (length of a degree at the equator)

111.319 km / 3600 (seconds in a degree) = 30.922 m (length of a second at the equator)

1" = 2.7777777777777777777777777777778e-4 degrees = 0.000277 degrees

So you're closer!

Melita

KenLucas

Melita,
It made my whole day to see your math in the last note! I was able to use it to arrive at my earlier calc, done with diff math, of
9.8m. It re assures, and makes sense, to me to get a realistic result, like 9.8 meters of error, for the tiff I geo referenced, instead of the really small number I misinterpreted. Thanks so much, Melita, from me and the other 3 gis analysts on our team, for helping us understand this process better. We have hundreds of historical tiffs to geo reference in the next year.

For the other folks following this discussion: let me try to sum up the process of interpreting the RMS error reported in the link table, using Melita's math :

determine that the figures under the X & Y map columns are lat and long coords The RMS error is then in decimal degrees.

Our RMS error * 3600 seconds = converts the decimal degrees figure to seconds. The RMS seconds * 30.922 = the error, in meters Of course, this is the error at the equator and should be converted to your lat, if you need the extra accuracy. I'll post a follow up thread, showing my RMS error corrected for my lat, after I figure out how to convert it.

I think the definition of the RMS error: the error between the coords of the link points set on the tiff and the coords of the link points set at the same position on the NAIP imagery. I hope someone will feel free to correct my definition, if I'm in error.

Ken

NeilAyres

As a rough approximation (ie on a sphere), just multiply your distance at the equator by the Cosine of the Latitude of where the data is to get the actual distance.
And to further underline Melita's point, you should always georeference into the correct projected coordinate system of the data. Not directly into GCS.
Good luck,
Neil

KenLucas

Neil,
Your 2 comments were really helpful, as we realized Melita's calcs were for a point on the equator, but didn't know how to convert to our latitude of 39 degrees. That makes a big diff, since our error drops from 32' to 9', which is what I expect from setting 8 decent link points. Plus, we just did another geo reference, using the native projection, as you suggested, UTM, zone 11, rather the WGS84, I used in my example and got an RMS of 15' with just 5 links. In that case, our individual link points are listed in the link table in utm coordinates, meters, and we have a Total RMS error = 8 Is a conversion required, or should we interpret this error as 8 meters?

We'd like to get the group's opinion about what the lowest RMS error / minimum link points should be for old tiffs?

I'm really grateful to you and Melita for assisting our team of 4 gis people. We have scores of old tiffs to geo reference for a water rights mapping project.
Ken

NeilAyres

Ken,
the to link coords and the reported rms error will always be in the referencing coordinate system. So the 8 will be metres.
As regards the question about accuracy, well I suppose that depends on the pixel size or original intended scale of the data.
If the referenced tiff has a pixel size of 0.5m, then 8m might seem a little off. If its 4 or 5m then you probably won't get any better.
This also depends on the accuracy / scale of the reference data as well of course.
Cheers,
Neil

KenLucas

Neil,
Following up on your last thread, I want to evaluate the pixel size of the imagery, we are using, and I'm not I'm interpreting the imagery's properties.

Cell Size = 1,1 Is this 1 meter by 1 meter? So, if my RMS is off 3 m that would not be considered serious error?
Pixel type= unsigned integer
pixel depth= 8 bit

By scale of the reference data, do you mean how far we are zoomed into the imagery when we do a link?
Ken

NeilAyres

Ken,
my comment about scale was just about the data source like 1:50k aerial imagery or whatnot.
Of course, when creating links it is probably good practice to always use the same zoom level scale consistently (say 1:5000, or 2000 etc). Again this will depend on the input data source.
This cell size you have :

Cell Size = 1,1 Is this 1 meter by 1 meter?

Is this before starting to georeference, when looking at the image properties in Catalog? If so this is just a pixel.
If it is after georeferencing, then I am a bit mystified. It seems hard to imagine how you would get the georeferencing so exact as to give you 1m instead of 1.02341999....m

KenLucas

Neil,

1,1 is the pixel size according the the properties report via ArcCatalog. Based on your earlier comment, about pixel size, that must be 1 x 1 meter. Thanks for explaining the scale. I looked, unscuccessfully, for scale in the properties. We know to be zoomed way in when setting link points.

We'd really like to get your opinion on the Update Geo referencing vs Rectify options. If you prefer to use the Rectify option what adjustments to you make the Rectify menu options dialog. We've researched this and don't know how to optimize the settings. If you don't use rectify then let us know, so we can be assured we are doing the right thing by just doing update geo referencing for our scores of tiffs.
Attached is a capture of the Rectify dialog box. I couldn't figure out how to insert it directly into this window.
Our gratitude for your continued help to us,
Ken and the 3 other DWR mapping section analysts

Capture2.JPG

NeilAyres

If these tiffs are going to be used in a mosaic or by other software, it is probably wise to rectify them into the new coordinate system.
Be aware, of course, that this creates a new image file and you will need sufficient disk storage for this.
As these are aerial photos, use cubic convolution as the resampling technique.
You could also select LZW compression which will make the tiffs a bit smaller.

But that window you posted has the cell size as 0.000006, which tells me that you are still georeffing into GCS degrees units.
Why is that? A proper projected coordinate system in metres or feet or whatever is your standard will be better.

N

KenLucas

Neil,
Since the start of the discussion we have since changed our procedure to georeffing into utm, meters and so, now, our pixel size = 0.65 Hopefully, those are meters. I don't think we should create a new file; which is what, I'm assuming, you're refering to as creating a new file via changing the existing rectified image's projection. We simply just changed the projection of the arcmap we are using to geo reff to utm, meters.
Thank you for suggesting how we could optimize the rectify the settings, we attempted to use the esri help, with no success.
Thank you.
Ken

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